Develop Reference

Trying to change properties in copied javascript array [duplicate]

I have an object x. I'd like to copy it as object y, such that changes to y do not modify x. I realized that copying objects derived from built-in JavaScript objects will result in extra, unwanted properties. This isn't a problem, since I'm copying one of my own literal-constructed objects.
How do I correctly clone a JavaScript object?

2022 update
There's a new JS standard called structured cloning. It works in many browsers (see Can I Use).
const clone = structuredClone(object);
Old answer
To do this for any object in JavaScript will not be simple or straightforward. You will run into the problem of erroneously picking up attributes from the object's prototype that should be left in the prototype and not copied to the new instance. If, for instance, you are adding a clone method to Object.prototype, as some answers depict, you will need to explicitly skip that attribute. But what if there are other additional methods added to Object.prototype, or other intermediate prototypes, that you don't know about? In that case, you will copy attributes you shouldn't, so you need to detect unforeseen, non-local attributes with the hasOwnProperty method.
In addition to non-enumerable attributes, you'll encounter a tougher problem when you try to copy objects that have hidden properties. For example, prototype is a hidden property of a function. Also, an object's prototype is referenced with the attribute __proto__, which is also hidden, and will not be copied by a for/in loop iterating over the source object's attributes. I think __proto__ might be specific to Firefox's JavaScript interpreter and it may be something different in other browsers, but you get the picture. Not everything is enumerable. You can copy a hidden attribute if you know its name, but I don't know of any way to discover it automatically.
Yet another snag in the quest for an elegant solution is the problem of setting up the prototype inheritance correctly. If your source object's prototype is Object, then simply creating a new general object with {} will work, but if the source's prototype is some descendant of Object, then you are going to be missing the additional members from that prototype which you skipped using the hasOwnProperty filter, or which were in the prototype, but weren't enumerable in the first place. One solution might be to call the source object's constructor property to get the initial copy object and then copy over the attributes, but then you still will not get non-enumerable attributes. For example, a Date object stores its data as a hidden member:
function clone(obj) {
if (null == obj || "object" != typeof obj) return obj;
var copy = obj.constructor();
for (var attr in obj) {
if (obj.hasOwnProperty(attr)) copy[attr] = obj[attr];
}
return copy;
}
var d1 = new Date();
/* Executes function after 5 seconds. */
setTimeout(function(){
var d2 = clone(d1);
alert("d1 = " + d1.toString() + "\nd2 = " + d2.toString());
}, 5000);
The date string for d1 will be 5 seconds behind that of d2. A way to make one Date the same as another is by calling the setTime method, but that is specific to the Date class. I don't think there is a bullet-proof general solution to this problem, though I would be happy to be wrong!
When I had to implement general deep copying I ended up compromising by assuming that I would only need to copy a plain Object, Array, Date, String, Number, or Boolean. The last 3 types are immutable, so I could perform a shallow copy and not worry about it changing. I further assumed that any elements contained in Object or Array would also be one of the 6 simple types in that list. This can be accomplished with code like the following:
function clone(obj) {
var copy;
// Handle the 3 simple types, and null or undefined
if (null == obj || "object" != typeof obj) return obj;
// Handle Date
if (obj instanceof Date) {
copy = new Date();
copy.setTime(obj.getTime());
return copy;
}
// Handle Array
if (obj instanceof Array) {
copy = [];
for (var i = 0, len = obj.length; i < len; i++) {
copy[i] = clone(obj[i]);
}
return copy;
}
// Handle Object
if (obj instanceof Object) {
copy = {};
for (var attr in obj) {
if (obj.hasOwnProperty(attr)) copy[attr] = clone(obj[attr]);
}
return copy;
}
throw new Error("Unable to copy obj! Its type isn't supported.");
}
The above function will work adequately for the 6 simple types I mentioned, as long as the data in the objects and arrays form a tree structure. That is, there isn't more than one reference to the same data in the object. For example:
// This would be cloneable:
var tree = {
"left" : { "left" : null, "right" : null, "data" : 3 },
"right" : null,
"data" : 8
};
// This would kind-of work, but you would get 2 copies of the
// inner node instead of 2 references to the same copy
var directedAcylicGraph = {
"left" : { "left" : null, "right" : null, "data" : 3 },
"data" : 8
};
directedAcyclicGraph["right"] = directedAcyclicGraph["left"];
// Cloning this would cause a stack overflow due to infinite recursion:
var cyclicGraph = {
"left" : { "left" : null, "right" : null, "data" : 3 },
"data" : 8
};
cyclicGraph["right"] = cyclicGraph;
It will not be able to handle any JavaScript object, but it may be sufficient for many purposes as long as you don't assume that it will just work for anything you throw at it.

If you do not use Dates, functions, undefined, regExp or Infinity within your object, a very simple one liner is JSON.parse(JSON.stringify(object)):
const a = {
string: 'string',
number: 123,
bool: false,
nul: null,
date: new Date(), // stringified
undef: undefined, // lost
inf: Infinity, // forced to 'null'
}
console.log(a);
console.log(typeof a.date); // Date object
const clone = JSON.parse(JSON.stringify(a));
console.log(clone);
console.log(typeof clone.date); // result of .toISOString()
This works for all kind of objects containing objects, arrays, strings, booleans and numbers.
See also this article about the structured clone algorithm of browsers which is used when posting messages to and from a worker. It also contains a function for deep cloning.

In ECMAScript 6 there is Object.assign method, which copies values of all enumerable own properties from one object to another. For example:
var x = {myProp: "value"};
var y = Object.assign({}, x);
But be aware this is a shallow copy - nested objects are still copied as reference.

With jQuery, you can shallow copy with extend:
var copiedObject = jQuery.extend({}, originalObject)
subsequent changes to the copiedObject will not affect the originalObject, and vice versa.
Or to make a deep copy:
var copiedObject = jQuery.extend(true, {}, originalObject)

Per MDN:
If you want shallow copy, use Object.assign({}, a)
For "deep" copy, use JSON.parse(JSON.stringify(a))
There is no need for external libraries but you need to check browser compatibility first.

An elegant way to clone a Javascript object in one line of code
An Object.assign method is part of the ECMAScript 2015 (ES6) standard and does exactly what you need.
var clone = Object.assign({}, obj);
The Object.assign() method is used to copy the values of all enumerable own properties from one or more source objects to a target object.
Read more...
The polyfill to support older browsers:
if (!Object.assign) {
Object.defineProperty(Object, 'assign', {
enumerable: false,
configurable: true,
writable: true,
value: function(target) {
'use strict';
if (target === undefined || target === null) {
throw new TypeError('Cannot convert first argument to object');
}
var to = Object(target);
for (var i = 1; i < arguments.length; i++) {
var nextSource = arguments[i];
if (nextSource === undefined || nextSource === null) {
continue;
}
nextSource = Object(nextSource);
var keysArray = Object.keys(nextSource);
for (var nextIndex = 0, len = keysArray.length; nextIndex < len; nextIndex++) {
var nextKey = keysArray[nextIndex];
var desc = Object.getOwnPropertyDescriptor(nextSource, nextKey);
if (desc !== undefined && desc.enumerable) {
to[nextKey] = nextSource[nextKey];
}
}
}
return to;
}
});
}

There are many answers, but none that mentions Object.create from ECMAScript 5, which admittedly does not give you an exact copy, but sets the source as the prototype of the new object.
Thus, this is not an exact answer to the question, but it is a one-line solution and thus elegant. And it works best for 2 cases:
Where such inheritance is useful (duh!)
Where the source object won't be modified, thus making the relation between the 2 objects a non issue.
Example:
var foo = { a : 1 };
var bar = Object.create(foo);
foo.a; // 1
bar.a; // 1
foo.a = 2;
bar.a; // 2 - prototype changed
bar.a = 3;
foo.a; // Still 2, since setting bar.a makes it an "own" property
Why do I consider this solution to be superior? It's native, thus no looping, no recursion. However, older browsers will need a polyfill.

There are several issues with most solutions on the internet. So I decided to make a follow-up, which includes, why the accepted answer shouldn't be accepted.
starting situation
I want to deep-copy a Javascript Object with all of its children and their children and so on. But since I'm not kind of a normal developer, my Object has normal properties, circular structures and even nested objects.
So let's create a circular structure and a nested object first.
function Circ() {
this.me = this;
}
function Nested(y) {
this.y = y;
}
Let's bring everything together in an Object named a.
var a = {
x: 'a',
circ: new Circ(),
nested: new Nested('a')
};
Next, we want to copy a into a variable named b and mutate it.
var b = a;
b.x = 'b';
b.nested.y = 'b';
You know what happened here because if not you wouldn't even land on this great question.
console.log(a, b);
a --> Object {
x: "b",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
b --> Object {
x: "b",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
Now let's find a solution.
JSON
The first attempt I tried was using JSON.
var b = JSON.parse( JSON.stringify( a ) );
b.x = 'b';
b.nested.y = 'b';
Don't waste too much time on it, you'll get TypeError: Converting circular structure to JSON.
Recursive copy (the accepted "answer")
Let's have a look at the accepted answer.
function cloneSO(obj) {
// Handle the 3 simple types, and null or undefined
if (null == obj || "object" != typeof obj) return obj;
// Handle Date
if (obj instanceof Date) {
var copy = new Date();
copy.setTime(obj.getTime());
return copy;
}
// Handle Array
if (obj instanceof Array) {
var copy = [];
for (var i = 0, len = obj.length; i < len; i++) {
copy[i] = cloneSO(obj[i]);
}
return copy;
}
// Handle Object
if (obj instanceof Object) {
var copy = {};
for (var attr in obj) {
if (obj.hasOwnProperty(attr)) copy[attr] = cloneSO(obj[attr]);
}
return copy;
}
throw new Error("Unable to copy obj! Its type isn't supported.");
}
Looks good, heh? It's a recursive copy of the object and handles other types as well, like Date, but that wasn't a requirement.
var b = cloneSO(a);
b.x = 'b';
b.nested.y = 'b';
Recursion and circular structures doesn't work well together... RangeError: Maximum call stack size exceeded
native solution
After arguing with my co-worker, my boss asked us what happened, and he found a simple solution after some googling. It's called Object.create.
var b = Object.create(a);
b.x = 'b';
b.nested.y = 'b';
This solution was added to Javascript some time ago and even handles circular structure.
console.log(a, b);
a --> Object {
x: "a",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
b --> Object {
x: "b",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
... and you see, it didn't work with the nested structure inside.
polyfill for the native solution
There's a polyfill for Object.create in the older browser just like the IE 8. It's something like recommended by Mozilla, and of course, it's not perfect and results in the same problem as the native solution.
function F() {};
function clonePF(o) {
F.prototype = o;
return new F();
}
var b = clonePF(a);
b.x = 'b';
b.nested.y = 'b';
I've put F outside the scope so we can have a look at what instanceof tells us.
console.log(a, b);
a --> Object {
x: "a",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
b --> F {
x: "b",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
console.log(typeof a, typeof b);
a --> object
b --> object
console.log(a instanceof Object, b instanceof Object);
a --> true
b --> true
console.log(a instanceof F, b instanceof F);
a --> false
b --> true
Same problem as the native solution, but a little bit worse output.
the better (but not perfect) solution
When digging around, I found a similar question (In Javascript, when performing a deep copy, how do I avoid a cycle, due to a property being "this"?) to this one, but with a way better solution.
function cloneDR(o) {
const gdcc = "__getDeepCircularCopy__";
if (o !== Object(o)) {
return o; // primitive value
}
var set = gdcc in o,
cache = o[gdcc],
result;
if (set && typeof cache == "function") {
return cache();
}
// else
o[gdcc] = function() { return result; }; // overwrite
if (o instanceof Array) {
result = [];
for (var i=0; i<o.length; i++) {
result[i] = cloneDR(o[i]);
}
} else {
result = {};
for (var prop in o)
if (prop != gdcc)
result[prop] = cloneDR(o[prop]);
else if (set)
result[prop] = cloneDR(cache);
}
if (set) {
o[gdcc] = cache; // reset
} else {
delete o[gdcc]; // unset again
}
return result;
}
var b = cloneDR(a);
b.x = 'b';
b.nested.y = 'b';
And let's have a look at the output...
console.log(a, b);
a --> Object {
x: "a",
circ: Object {
me: Object { ... }
},
nested: Object {
y: "a"
}
}
b --> Object {
x: "b",
circ: Object {
me: Object { ... }
},
nested: Object {
y: "b"
}
}
console.log(typeof a, typeof b);
a --> object
b --> object
console.log(a instanceof Object, b instanceof Object);
a --> true
b --> true
console.log(a instanceof F, b instanceof F);
a --> false
b --> false
The requirements are matched, but there are still some smaller issues, including changing the instance of nested and circ to Object.
The structure of trees that share a leaf won't be copied, they will become two independent leaves:
[Object] [Object]
/ \ / \
/ \ / \
|/_ _\| |/_ _\|
[Object] [Object] ===> [Object] [Object]
\ / | |
\ / | |
_\| |/_ \|/ \|/
[Object] [Object] [Object]
conclusion
The last solution using recursion and a cache, may not be the best, but it's a real deep-copy of the object. It handles simple properties, circular structures and nested object, but it will mess up the instance of them while cloning.
jsfiddle

If you're okay with a shallow copy, the underscore.js library has a clone method.
y = _.clone(x);
or you can extend it like
copiedObject = _.extend({},originalObject);

OK, imagine you have this object below and you want to clone it:
let obj = {a:1, b:2, c:3}; //ES6
or
var obj = {a:1, b:2, c:3}; //ES5
the answer is mainly depeneds on which ECMAscript you using, in ES6+, you can simply use Object.assign to do the clone:
let cloned = Object.assign({}, obj); //new {a:1, b:2, c:3};
or using spread operator like this:
let cloned = {...obj}; //new {a:1, b:2, c:3};
But if you using ES5, you can use few methods, but the JSON.stringify, just make sure you not using for a big chunk of data to copy, but it could be one line handy way in many cases, something like this:
let cloned = JSON.parse(JSON.stringify(obj));
//new {a:1, b:2, c:3};, can be handy, but avoid using on big chunk of data over and over

Update 06 July 2020
There are three (3) ways to clone objects in JavaScript. As objects in JavaScript are reference values, you can't simply just copy using the =.
The ways are:
const food = { food: 'apple', drink: 'milk' }
// 1. Using the "Spread"
// ------------------
{ ...food }
// 2. Using "Object.assign"
// ------------------
Object.assign({}, food)
// 3. "JSON"
// ------------------
JSON.parse(JSON.stringify(food))
// RESULT:
// { food: 'apple', drink: 'milk' }
This can be used as a reference summary.

One particularly inelegant solution is to use JSON encoding to make deep copies of objects that do not have member methods. The methodology is to JSON encode your target object, then by decoding it, you get the copy you are looking for. You can decode as many times as you want to make as many copies as you need.
Of course, functions do not belong in JSON, so this only works for objects without member methods.
This methodology was perfect for my use case, since I'm storing JSON blobs in a key-value store, and when they are exposed as objects in a JavaScript API, each object actually contains a copy of the original state of the object so we can calculate the delta after the caller has mutated the exposed object.
var object1 = {key:"value"};
var object2 = object1;
object2 = JSON.stringify(object1);
object2 = JSON.parse(object2);
object2.key = "a change";
console.log(object1);// returns value

You can simply use a spread property to copy an object without references. But be careful (see comments), the 'copy' is just on the lowest object/array level. Nested properties are still references!
Complete clone:
let x = {a: 'value1'}
let x2 = {...x}
// => mutate without references:
x2.a = 'value2'
console.log(x.a) // => 'value1'
Clone with references on second level:
const y = {a: {b: 'value3'}}
const y2 = {...y}
// => nested object is still a references:
y2.a.b = 'value4'
console.log(y.a.b) // => 'value4'
JavaScript actually does not support deep clones natively. Use an utility function. For example Ramda:
http://ramdajs.com/docs/#clone

const objClone = { ...obj };
Be aware that nested objects are still copied as a reference.

From this article: How to copy arrays and objects in Javascript by Brian Huisman:
Object.prototype.clone = function() {
var newObj = (this instanceof Array) ? [] : {};
for (var i in this) {
if (i == 'clone') continue;
if (this[i] && typeof this[i] == "object") {
newObj[i] = this[i].clone();
} else newObj[i] = this[i]
} return newObj;
};

For those using AngularJS, there is also direct method for cloning or extending of the objects in this library.
var destination = angular.copy(source);
or
angular.copy(source, destination);
More in angular.copy documentation...

function clone(obj) {
if(obj == null || typeof(obj) != 'object')
return obj;
var temp = new obj.constructor();
for(var key in obj)
temp[key] = clone(obj[key]);
return temp;
}

A.Levy's answer is almost complete, here is my little contribution: there is a way how to handle recursive references, see this line
if(this[attr]==this) copy[attr] = copy;
If the object is XML DOM element, we must use cloneNode instead
if(this.cloneNode) return this.cloneNode(true);
Inspired by A.Levy's exhaustive study and Calvin's prototyping approach, I offer this solution:
Object.prototype.clone = function() {
if(this.cloneNode) return this.cloneNode(true);
var copy = this instanceof Array ? [] : {};
for(var attr in this) {
if(typeof this[attr] == "function" || this[attr]==null || !this[attr].clone)
copy[attr] = this[attr];
else if(this[attr]==this) copy[attr] = copy;
else copy[attr] = this[attr].clone();
}
return copy;
}
Date.prototype.clone = function() {
var copy = new Date();
copy.setTime(this.getTime());
return copy;
}
Number.prototype.clone =
Boolean.prototype.clone =
String.prototype.clone = function() {
return this;
}
See also Andy Burke's note in the answers.

Performance
Today 2020.04.30 I perform tests of chosen solutions on Chrome v81.0, Safari v13.1 and Firefox v75.0 on MacOs High Sierra v10.13.6.
I focus on speed of copy DATA (object with simple type fields, not methods etc.). The solutions A-I can make only shallow copy, solutions J-U can make deep copy.
Results for shallow copy
solution {...obj} (A) is fastest on chrome and firefox and medium fast on safari
solution based on Object.assign (B) is fast on all browsers
jQuery (E) and lodash (F,G,H) solutions are medium/quite fast
solution JSON.parse/stringify (K) is quite slow
solutions D and U are slow on all browsers
Results for deep copy
solution Q is fastest on all browsers
jQuery (L) and lodash (J) are medium fast
solution JSON.parse/stringify (K) is quite slow
solution U is slowest on all browsers
lodash (J) and solution U crash on Chrome for 1000 level deep object
Details
For choosen solutions:
A
B
C(my)
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U,
I perform 4 tests
shallow-small: object with 10 non-nested fields - you can run it HERE
shallow-big: object with 1000 non-nested fields - you can run it HERE
deep-small: object with 10 levels-nested fields - you can run it HERE
deep-big: object with 1000 levels-nested fields - you can run it HERE
Objects used in tests are show in below snippet
let obj_ShallowSmall = {
field0: false,
field1: true,
field2: 1,
field3: 0,
field4: null,
field5: [],
field6: {},
field7: "text7",
field8: "text8",
}
let obj_DeepSmall = {
level0: {
level1: {
level2: {
level3: {
level4: {
level5: {
level6: {
level7: {
level8: {
level9: [[[[[[[[[['abc']]]]]]]]]],
}}}}}}}}},
};
let obj_ShallowBig = Array(1000).fill(0).reduce((a,c,i) => (a['field'+i]=getField(i),a) ,{});
let obj_DeepBig = genDeepObject(1000);
// ------------------
// Show objects
// ------------------
console.log('obj_ShallowSmall:',JSON.stringify(obj_ShallowSmall));
console.log('obj_DeepSmall:',JSON.stringify(obj_DeepSmall));
console.log('obj_ShallowBig:',JSON.stringify(obj_ShallowBig));
console.log('obj_DeepBig:',JSON.stringify(obj_DeepBig));
// ------------------
// HELPERS
// ------------------
function getField(k) {
let i=k%10;
if(i==0) return false;
if(i==1) return true;
if(i==2) return k;
if(i==3) return 0;
if(i==4) return null;
if(i==5) return [];
if(i==6) return {};
if(i>=7) return "text"+k;
}
function genDeepObject(N) {
// generate: {level0:{level1:{...levelN: {end:[[[...N-times...['abc']...]]] }}}...}}}
let obj={};
let o=obj;
let arr = [];
let a=arr;
for(let i=0; i<N; i++) {
o['level'+i]={};
o=o['level'+i];
let aa=[];
a.push(aa);
a=aa;
}
a[0]='abc';
o['end']=arr;
return obj;
}
Below snippet presents tested solutions and shows differences between them
function A(obj) {
return {...obj}
}
function B(obj) {
return Object.assign({}, obj);
}
function C(obj) {
return Object.keys(obj).reduce( (a,c) => (a[c]=obj[c], a), {})
}
function D(obj) {
let copyOfObject = {};
Object.defineProperties(copyOfObject, Object.getOwnPropertyDescriptors(obj));
return copyOfObject;
}
function E(obj) {
return jQuery.extend({}, obj) // shallow
}
function F(obj) {
return _.clone(obj);
}
function G(obj) {
return _.clone(obj,true);
}
function H(obj) {
return _.extend({},obj);
}
function I(obj) {
if (null == obj || "object" != typeof obj) return obj;
var copy = obj.constructor();
for (var attr in obj) {
if (obj.hasOwnProperty(attr)) copy[attr] = obj[attr];
}
return copy;
}
function J(obj) {
return _.cloneDeep(obj,true);
}
function K(obj) {
return JSON.parse(JSON.stringify(obj));
}
function L(obj) {
return jQuery.extend(true, {}, obj) // deep
}
function M(obj) {
if(obj == null || typeof(obj) != 'object')
return obj;
var temp = new obj.constructor();
for(var key in obj)
temp[key] = M(obj[key]);
return temp;
}
function N(obj) {
let EClone = function(obj) {
var newObj = (obj instanceof Array) ? [] : {};
for (var i in obj) {
if (i == 'EClone') continue;
if (obj[i] && typeof obj[i] == "object") {
newObj[i] = EClone(obj[i]);
} else newObj[i] = obj[i]
} return newObj;
};
return EClone(obj);
};
function O(obj) {
if (obj == null || typeof obj != "object") return obj;
if (obj.constructor != Object && obj.constructor != Array) return obj;
if (obj.constructor == Date || obj.constructor == RegExp || obj.constructor == Function ||
obj.constructor == String || obj.constructor == Number || obj.constructor == Boolean)
return new obj.constructor(obj);
let to = new obj.constructor();
for (var name in obj)
{
to[name] = typeof to[name] == "undefined" ? O(obj[name], null) : to[name];
}
return to;
}
function P(obj) {
function clone(target, source){
for(let key in source){
// Use getOwnPropertyDescriptor instead of source[key] to prevent from trigering setter/getter.
let descriptor = Object.getOwnPropertyDescriptor(source, key);
if(descriptor.value instanceof String){
target[key] = new String(descriptor.value);
}
else if(descriptor.value instanceof Array){
target[key] = clone([], descriptor.value);
}
else if(descriptor.value instanceof Object){
let prototype = Reflect.getPrototypeOf(descriptor.value);
let cloneObject = clone({}, descriptor.value);
Reflect.setPrototypeOf(cloneObject, prototype);
target[key] = cloneObject;
}
else {
Object.defineProperty(target, key, descriptor);
}
}
let prototype = Reflect.getPrototypeOf(source);
Reflect.setPrototypeOf(target, prototype);
return target;
}
return clone({},obj);
}
function Q(obj) {
var copy;
// Handle the 3 simple types, and null or undefined
if (null == obj || "object" != typeof obj) return obj;
// Handle Date
if (obj instanceof Date) {
copy = new Date();
copy.setTime(obj.getTime());
return copy;
}
// Handle Array
if (obj instanceof Array) {
copy = [];
for (var i = 0, len = obj.length; i < len; i++) {
copy[i] = Q(obj[i]);
}
return copy;
}
// Handle Object
if (obj instanceof Object) {
copy = {};
for (var attr in obj) {
if (obj.hasOwnProperty(attr)) copy[attr] = Q(obj[attr]);
}
return copy;
}
throw new Error("Unable to copy obj! Its type isn't supported.");
}
function R(obj) {
const gdcc = "__getDeepCircularCopy__";
if (obj !== Object(obj)) {
return obj; // primitive value
}
var set = gdcc in obj,
cache = obj[gdcc],
result;
if (set && typeof cache == "function") {
return cache();
}
// else
obj[gdcc] = function() { return result; }; // overwrite
if (obj instanceof Array) {
result = [];
for (var i=0; i<obj.length; i++) {
result[i] = R(obj[i]);
}
} else {
result = {};
for (var prop in obj)
if (prop != gdcc)
result[prop] = R(obj[prop]);
else if (set)
result[prop] = R(cache);
}
if (set) {
obj[gdcc] = cache; // reset
} else {
delete obj[gdcc]; // unset again
}
return result;
}
function S(obj) {
const cache = new WeakMap(); // Map of old - new references
function copy(object) {
if (typeof object !== 'object' ||
object === null ||
object instanceof HTMLElement
)
return object; // primitive value or HTMLElement
if (object instanceof Date)
return new Date().setTime(object.getTime());
if (object instanceof RegExp)
return new RegExp(object.source, object.flags);
if (cache.has(object))
return cache.get(object);
const result = object instanceof Array ? [] : {};
cache.set(object, result); // store reference to object before the recursive starts
if (object instanceof Array) {
for(const o of object) {
result.push(copy(o));
}
return result;
}
const keys = Object.keys(object);
for (const key of keys)
result[key] = copy(object[key]);
return result;
}
return copy(obj);
}
function T(obj){
var clonedObjectsArray = [];
var originalObjectsArray = []; //used to remove the unique ids when finished
var next_objid = 0;
function objectId(obj) {
if (obj == null) return null;
if (obj.__obj_id == undefined){
obj.__obj_id = next_objid++;
originalObjectsArray[obj.__obj_id] = obj;
}
return obj.__obj_id;
}
function cloneRecursive(obj) {
if (null == obj || typeof obj == "string" || typeof obj == "number" || typeof obj == "boolean") return obj;
// Handle Date
if (obj instanceof Date) {
var copy = new Date();
copy.setTime(obj.getTime());
return copy;
}
// Handle Array
if (obj instanceof Array) {
var copy = [];
for (var i = 0; i < obj.length; ++i) {
copy[i] = cloneRecursive(obj[i]);
}
return copy;
}
// Handle Object
if (obj instanceof Object) {
if (clonedObjectsArray[objectId(obj)] != undefined)
return clonedObjectsArray[objectId(obj)];
var copy;
if (obj instanceof Function)//Handle Function
copy = function(){return obj.apply(this, arguments);};
else
copy = {};
clonedObjectsArray[objectId(obj)] = copy;
for (var attr in obj)
if (attr != "__obj_id" && obj.hasOwnProperty(attr))
copy[attr] = cloneRecursive(obj[attr]);
return copy;
}
throw new Error("Unable to copy obj! Its type isn't supported.");
}
var cloneObj = cloneRecursive(obj);
//remove the unique ids
for (var i = 0; i < originalObjectsArray.length; i++)
{
delete originalObjectsArray[i].__obj_id;
};
return cloneObj;
}
function U(obj) {
/*
Deep copy objects by value rather than by reference,
exception: `Proxy`
*/
const seen = new WeakMap()
return clone(obj)
function defineProp(object, key, descriptor = {}, copyFrom = {}) {
const { configurable: _configurable, writable: _writable }
= Object.getOwnPropertyDescriptor(object, key)
|| { configurable: true, writable: true }
const test = _configurable // Can redefine property
&& (_writable === undefined || _writable) // Can assign to property
if (!test || arguments.length <= 2) return test
const basisDesc = Object.getOwnPropertyDescriptor(copyFrom, key)
|| { configurable: true, writable: true } // Custom…
|| {}; // …or left to native default settings
["get", "set", "value", "writable", "enumerable", "configurable"]
.forEach(attr =>
descriptor[attr] === undefined &&
(descriptor[attr] = basisDesc[attr])
)
const { get, set, value, writable, enumerable, configurable }
= descriptor
return Object.defineProperty(object, key, {
enumerable, configurable, ...get || set
? { get, set } // Accessor descriptor
: { value, writable } // Data descriptor
})
}
function clone(object) {
if (object !== Object(object)) return object /*
—— Check if the object belongs to a primitive data type */
if (object instanceof Node) return object.cloneNode(true) /*
—— Clone DOM trees */
let _object // The clone of object
switch (object.constructor) {
case Array:
case Object:
_object = cloneObject(object)
break
case Date:
_object = new Date(+object)
break
case Function:
const fnStr = String(object)
_object = new Function("return " +
(/^(?!function |[^{]+?=>)[^(]+?\(/.test(fnStr)
? "function " : ""
) + fnStr
)()
copyPropDescs(_object, object)
break
case RegExp:
_object = new RegExp(object)
break
default:
switch (Object.prototype.toString.call(object.constructor)) {
// // Stem from:
case "[object Function]": // `class`
case "[object Undefined]": // `Object.create(null)`
_object = cloneObject(object)
break
default: // `Proxy`
_object = object
}
}
return _object
}
function cloneObject(object) {
if (seen.has(object)) return seen.get(object) /*
—— Handle recursive references (circular structures) */
const _object = Array.isArray(object)
? []
: Object.create(Object.getPrototypeOf(object)) /*
—— Assign [[Prototype]] for inheritance */
seen.set(object, _object) /*
—— Make `_object` the associative mirror of `object` */
Reflect.ownKeys(object).forEach(key =>
defineProp(_object, key, { value: clone(object[key]) }, object)
)
return _object
}
function copyPropDescs(target, source) {
Object.defineProperties(target,
Object.getOwnPropertyDescriptors(source)
)
}
}
// ------------------------
// Test properties
// ------------------------
console.log(` shallow deep func circ undefined date RegExp bigInt`)
log(A);
log(B);
log(C);
log(D);
log(E);
log(F);
log(G);
log(H);
log(I);
log(J);
log(K);
log(L);
log(M);
log(N);
log(O);
log(P);
log(Q);
log(R);
log(S);
log(T);
log(U);
console.log(` shallow deep func circ undefined date RegExp bigInt
----
LEGEND:
shallow - solution create shallow copy
deep - solution create deep copy
func - solution copy functions
circ - solution can copy object with circular references
undefined - solution copy fields with undefined value
date - solution can copy date
RegExp - solution can copy fields with regular expressions
bigInt - solution can copy BigInt
`)
// ------------------------
// Helper functions
// ------------------------
function deepCompare(obj1,obj2) {
return JSON.stringify(obj1)===JSON.stringify(obj2);
}
function getCase() { // pure data case
return {
undef: undefined,
bool: true, num: 1, str: "txt1",
e1: null, e2: [], e3: {}, e4: 0, e5: false,
arr: [ false, 2, "txt3", null, [], {},
[ true,4,"txt5",null, [], {}, [true,6,"txt7",null,[],{} ],
{bool: true,num: 8, str: "txt9", e1:null, e2:[] ,e3:{} ,e4: 0, e5: false}
],
{bool: true,num: 10, str: "txt11", e1:null, e2:[] ,e3:{} ,e4: 0, e5: false}
],
obj: {
bool: true, num: 12, str: "txt13",
e1: null, e2: [], e3: {}, e4: 0, e5: false,
arr: [true,14,"txt15",null,[],{} ],
obj: {
bool: true, num: 16, str: "txt17",
e1: null, e2: [], e3: {}, e4: 0, e5: false,
arr: [true,18,"txt19",null,[],{} ],
obj: {bool: true,num: 20, str: "txt21", e1:null, e2:[] ,e3:{} ,e4: 0, e5: false}
}
}
};
}
function check(org, copy, field, newValue) {
copy[field] = newValue;
return deepCompare(org,copy);
}
function testFunc(f) {
let o = { a:1, fun: (i,j)=> i+j };
let c = f(o);
let val = false
try{
val = c.fun(3,4)==7;
} catch(e) { }
return val;
}
function testCirc(f) {
function Circ() {
this.me = this;
}
var o = {
x: 'a',
circ: new Circ(),
obj_circ: null,
};
o.obj_circ = o;
let val = false;
try{
let c = f(o);
val = (o.obj_circ == o) && (o.circ == o.circ.me);
} catch(e) { }
return val;
}
function testRegExp(f) {
let o = {
re: /a[0-9]+/,
};
let val = false;
try{
let c = f(o);
val = (String(c.re) == String(/a[0-9]+/));
} catch(e) { }
return val;
}
function testDate(f) {
let o = {
date: new Date(),
};
let val = false;
try{
let c = f(o);
val = (+new Date(c.date) == +new Date(o.date));
} catch(e) { }
return val;
}
function testBigInt(f) {
let val = false;
try{
let o = {
big: 123n,
};
let c = f(o);
val = o.big == c.big;
} catch(e) { }
return val;
}
function log(f) {
let o = getCase(); // orginal object
let oB = getCase(); // "backup" used for shallow valid test
let c1 = f(o); // copy 1 for reference
let c2 = f(o); // copy 2 for test shallow values
let c3 = f(o); // copy 3 for test deep values
let is_proper_copy = deepCompare(c1,o); // shoud be true
// shallow changes
let testShallow =
[ ['bool',false],['num',666],['str','xyz'],['arr',[]],['obj',{}] ]
.reduce((acc,curr)=> acc && check(c1,c2,curr[0], curr[1]), true );
// should be true (original object shoud not have changed shallow fields)
let is_valid = deepCompare(o,oB);
// deep test (intruduce some change)
if (c3.arr[6]) c3.arr[6][7].num = 777;
let diff_shallow = !testShallow; // shoud be true (shallow field was copied)
let diff_deep = !deepCompare(c1,c3); // shoud be true (deep field was copied)
let can_copy_functions = testFunc(f);
let can_copy_circular = testCirc(f);
let can_copy_regexp = testRegExp(f);
let can_copy_date = testDate(f);
let can_copy_bigInt = testBigInt(f);
let has_undefined = 'undef' in c1; // field with undefined value is copied?
let is_ok = is_valid && is_proper_copy;
let b=(bool) => (bool+'').padEnd(5,' '); // bool value to formated string
testFunc(f);
if(is_ok) {
console.log(`${f.name} ${b(diff_shallow)} ${b(diff_deep)} ${b(can_copy_functions)} ${b(can_copy_circular)} ${b(has_undefined)} ${b(can_copy_date)} ${b(can_copy_regexp)} ${b(can_copy_bigInt)}`)
} else {
console.log(`${f.name}: INVALID ${is_valid} ${is_proper_copy}`,{c1})
}
}
<script src="https://code.jquery.com/jquery-3.5.0.min.js" integrity="sha256-xNzN2a4ltkB44Mc/Jz3pT4iU1cmeR0FkXs4pru/JxaQ=" crossorigin="anonymous"></script>
<script src="https://cdn.jsdelivr.net/npm/lodash#4.17.15/lodash.min.js"></script>
This snippet only presents tested solutions and show differences between them (but it no make performence tests)
Below there are example results for Chrome for shallow-big object

Using Lodash:
var y = _.clone(x, true);

In ES-6 you can simply use Object.assign(...).
Ex:
let obj = {person: 'Thor Odinson'};
let clone = Object.assign({}, obj);
A good reference is here:
https://googlechrome.github.io/samples/object-assign-es6/

Interested in cloning simple objects:
JSON.parse(JSON.stringify(json_original));
Source : How to copy JavaScript object to new variable NOT by reference?

let clone = Object.assign( Object.create( Object.getPrototypeOf(obj)), obj)
ES6 solution if you want to (shallow) clone a class instance and not just a property object.

Structured Cloning
2022 update: The structuredClone() global function is already available in Node 17, Deno 1.14, and most major browsers (see Can I Use).
You can use the same structured clone mechanism that the HTML standard includes for sending data between realms.
const clone = structuredClone(original);
See the other answer for more details.

You can clone an object and remove any reference from the previous one using a single line of code. Simply do:
var obj1 = { text: 'moo1' };
var obj2 = Object.create(obj1); // Creates a new clone without references
obj2.text = 'moo2'; // Only updates obj2's text property
console.log(obj1, obj2); // Outputs: obj1: {text:'moo1'}, obj2: {text:'moo2'}
For browsers / engines that do not currently support Object.create you can use this polyfill:
// Polyfill Object.create if it does not exist
if (!Object.create) {
Object.create = function (o) {
var F = function () {};
F.prototype = o;
return new F();
};
}

New answer to an old question! If you have the pleasure of having using ECMAScript 2016 (ES6) with Spread Syntax, it's easy.
keepMeTheSame = {first: "Me!", second: "You!"};
cloned = {...keepMeTheSame}
This provides a clean method for a shallow copy of an object. Making a deep copy, meaning makign a new copy of every value in every recursively nested object, requires on of the heavier solutions above.
JavaScript keeps evolving.

I think there is a simple and working answer. In deep copying there are two concerns:
Keep properties independent to each other.
And keep the methods alive on cloned object.
So I think one simple solution will be to first serialize and deserialize and then do an assign on it to copy functions too.
let deepCloned = JSON.parse(JSON.stringify(source));
let merged = Object.assign({}, source);
Object.assign(merged, deepCloned);
Although this question has many answers, I hope this one helps too.

For a deep copy and clone, JSON.stringify then JSON.parse the object:
obj = { a: 0 , b: { c: 0}};
let deepClone = JSON.parse(JSON.stringify(obj));
obj.a = 5;
obj.b.c = 5;
console.log(JSON.stringify(deepClone)); // { a: 0, b: { c: 0}}

(The following was mainly an integration of #Maciej Bukowski, #A. Levy, #Jan Turoň, #Redu's answers, and #LeviRoberts, #RobG's comments, many thanks to them!!!)
Deep copy? — YES! (mostly);
Shallow copy? — NO! (except Proxy).
I sincerely welcome everyone to test clone().
In addition, defineProp() is designed to easily and quickly (re)define or copy any type of descriptor.
Function
function clone(object) {
/*
Deep copy objects by value rather than by reference,
exception: `Proxy`
*/
const seen = new WeakMap()
return clone(object)
function clone(object) {
if (object !== Object(object)) return object /*
—— Check if the object belongs to a primitive data type */
if (object instanceof Node) return object.cloneNode(true) /*
—— Clone DOM trees */
let _object // The clone of object
switch (object.constructor) {
case Array:
case Object:
_object = cloneObject(object)
break
case Date:
_object = new Date(+object)
break
case Function:
_object = copyFn(object)
break
case RegExp:
_object = new RegExp(object)
break
default:
switch (Object.prototype.toString.call(object.constructor)) {
// // Stem from:
case "[object Function]":
switch (object[Symbol.toStringTag]) {
case undefined:
_object = cloneObject(object) // `class`
break
case "AsyncFunction":
case "GeneratorFunction":
case "AsyncGeneratorFunction":
_object = copyFn(object)
break
default:
_object = object
}
break
case "[object Undefined]": // `Object.create(null)`
_object = cloneObject(object)
break
default:
_object = object // `Proxy`
}
}
return _object
}
function cloneObject(object) {
if (seen.has(object)) return seen.get(object) /*
—— Handle recursive references (circular structures) */
const _object = Array.isArray(object)
? []
: Object.create(Object.getPrototypeOf(object)) /*
—— Assign [[Prototype]] for inheritance */
seen.set(object, _object) /*
—— Make `_object` the associative mirror of `object` */
Reflect.ownKeys(object).forEach(key =>
defineProp(_object, key, { value: clone(object[key]) }, object)
)
return _object
}
}
function copyPropDescs(target, source) {
Object.defineProperties(target,
Object.getOwnPropertyDescriptors(source)
)
}
function convertFnToStr(fn) {
let fnStr = String(fn)
if (fn.name.startsWith("[")) // isSymbolKey
fnStr = fnStr.replace(/\[Symbol\..+?\]/, '')
fnStr = /^(?!(async )?(function\b|[^{]+?=>))[^(]+?\(/.test(fnStr)
? fnStr.replace(/^(async )?(\*)?/, "$1function$2 ") : fnStr
return fnStr
}
function copyFn(fn) {
const newFn = new Function(`return ${convertFnToStr(fn)}`)()
copyPropDescs(newFn, fn)
return newFn
}
function defineProp(object, key, descriptor = {}, copyFrom = {}) {
const { configurable: _configurable, writable: _writable }
= Object.getOwnPropertyDescriptor(object, key)
|| { configurable: true, writable: true }
const test = _configurable // Can redefine property
&& (_writable === undefined || _writable) // Can assign to property
if (!test || arguments.length <= 2) return test
const basisDesc = Object.getOwnPropertyDescriptor(copyFrom, key)
|| { configurable: true, writable: true } // Custom…
|| {}; // …or left to native default settings
["get", "set", "value", "writable", "enumerable", "configurable"]
.forEach(attr =>
descriptor[attr] === undefined &&
(descriptor[attr] = basisDesc[attr])
)
const { get, set, value, writable, enumerable, configurable }
= descriptor
return Object.defineProperty(object, key, {
enumerable, configurable, ...get || set
? { get, set } // Accessor descriptor
: { value, writable } // Data descriptor
})
}
// Tests
const obj0 = {
u: undefined,
nul: null,
t: true,
num: 9,
str: "",
sym: Symbol("symbol"),
[Symbol("e")]: Math.E,
arr: [[0], [1, 2]],
d: new Date(),
re: /f/g,
get g() { return 0 },
o: {
n: 0,
o: { f: function (...args) { } }
},
f: {
getAccessorStr(object) {
return []
.concat(...
Object.values(Object.getOwnPropertyDescriptors(object))
.filter(desc => desc.writable === undefined)
.map(desc => Object.values(desc))
)
.filter(prop => typeof prop === "function")
.map(String)
},
f0: function f0() { },
f1: function () { },
f2: a => a / (a + 1),
f3: () => 0,
f4(params) { return param => param + params },
f5: (a, b) => ({ c = 0 } = {}) => a + b + c
}
}
defineProp(obj0, "s", { set(v) { this._s = v } })
defineProp(obj0.arr, "tint", { value: { is: "non-enumerable" } })
obj0.arr[0].name = "nested array"
let obj1 = clone(obj0)
obj1.o.n = 1
obj1.o.o.g = function g(a = 0, b = 0) { return a + b }
obj1.arr[1][1] = 3
obj1.d.setTime(+obj0.d + 60 * 1000)
obj1.arr.tint.is = "enumerable? no"
obj1.arr[0].name = "a nested arr"
defineProp(obj1, "s", { set(v) { this._s = v + 1 } })
defineProp(obj1.re, "multiline", { value: true })
console.log("\n\n" + "-".repeat(2 ** 6))
console.log(">:>: Test - Routinely")
console.log("obj0:\n ", JSON.stringify(obj0))
console.log("obj1:\n ", JSON.stringify(obj1))
console.log()
console.log("obj0:\n ", obj0)
console.log("obj1:\n ", obj1)
console.log()
console.log("obj0\n ",
".arr.tint:", obj0.arr.tint, "\n ",
".arr[0].name:", obj0.arr[0].name
)
console.log("obj1\n ",
".arr.tint:", obj1.arr.tint, "\n ",
".arr[0].name:", obj1.arr[0].name
)
console.log()
console.log("Accessor-type descriptor\n ",
"of obj0:", obj0.f.getAccessorStr(obj0), "\n ",
"of obj1:", obj1.f.getAccessorStr(obj1), "\n ",
"set (obj0 & obj1) .s :", obj0.s = obj1.s = 0, "\n ",
" → (obj0 , obj1) ._s:", obj0._s, ",", obj1._s
)
console.log("—— obj0 has not been interfered.")
console.log("\n\n" + "-".repeat(2 ** 6))
console.log(">:>: Test - More kinds of functions")
const fnsForTest = {
f(_) { return _ },
func: _ => _,
aFunc: async _ => _,
async function() { },
async asyncFunc() { },
aFn: async function () { },
*gen() { },
async *asyncGen() { },
aG1: async function* () { },
aG2: async function* gen() { },
*[Symbol.iterator]() { yield* Object.keys(this) }
}
console.log(Reflect.ownKeys(fnsForTest).map(k =>
`${String(k)}:
${fnsForTest[k].name}-->
${String(fnsForTest[k])}`
).join("\n"))
const normedFnsStr = `{
f: function f(_) { return _ },
func: _ => _,
aFunc: async _ => _,
function: async function() { },
asyncFunc: async function asyncFunc() { },
aFn: async function () { },
gen: function* gen() { },
asyncGen: async function* asyncGen() { },
aG1: async function* () { },
aG2: async function* gen() { },
[Symbol.iterator]: function* () { yield* Object.keys(this) }
}`
const copiedFnsForTest = clone(fnsForTest)
console.log("fnsForTest:", fnsForTest)
console.log("fnsForTest (copied):", copiedFnsForTest)
console.log("fnsForTest (normed str):", eval(`(${normedFnsStr})`))
console.log("Comparison of fnsForTest and its clone:",
Reflect.ownKeys(fnsForTest).map(k =>
[k, fnsForTest[k] === copiedFnsForTest[k]]
)
)
console.log("\n\n" + "-".repeat(2 ** 6))
console.log(">:>: Test - Circular structures")
obj0.o.r = {}
obj0.o.r.recursion = obj0.o
obj0.arr[1] = obj0.arr
obj1 = clone(obj0)
console.log("obj0:\n ", obj0)
console.log("obj1:\n ", obj1)
console.log("Clear obj0's recursion:",
obj0.o.r.recursion = null, obj0.arr[1] = 1
)
console.log(
"obj0\n ",
".o.r:", obj0.o.r, "\n ",
".arr:", obj0.arr
)
console.log(
"obj1\n ",
".o.r:", obj1.o.r, "\n ",
".arr:", obj1.arr
)
console.log("—— obj1 has not been interfered.")
console.log("\n\n" + "-".repeat(2 ** 6))
console.log(">:>: Test - Classes")
class Person {
constructor(name) {
this.name = name
}
}
class Boy extends Person { }
Boy.prototype.sex = "M"
const boy0 = new Boy
boy0.hobby = { sport: "spaceflight" }
const boy1 = clone(boy0)
boy1.hobby.sport = "superluminal flight"
boy0.name = "one"
boy1.name = "neo"
console.log("boy0:\n ", boy0)
console.log("boy1:\n ", boy1)
console.log("boy1's prototype === boy0's:",
Object.getPrototypeOf(boy1) === Object.getPrototypeOf(boy0)
)
References
Object.create() | MDN
Object.defineProperties() | MDN
Enumerability and ownership of properties | MDN
TypeError: cyclic object value | MDN
Language tricks used
Conditionally add prop to object

Use lodash _.cloneDeep().
Shallow Copy: lodash _.clone()
A shallow copy can be made by simply copying the reference.
let obj1 = {
a: 0,
b: {
c: 0,
e: {
f: 0
}
}
};
let obj3 = _.clone(obj1);
obj1.a = 4;
obj1.b.c = 4;
obj1.b.e.f = 100;
console.log(JSON.stringify(obj1));
//{"a":4,"b":{"c":4,"e":{"f":100}}}
console.log(JSON.stringify(obj3));
//{"a":0,"b":{"c":4,"e":{"f":100}}}
Deep Copy: lodash _.cloneDeep()
fields are dereferenced: rather than references to objects being copied
let obj1 = {
a: 0,
b: {
c: 0,
e: {
f: 0
}
}
};
let obj3 = _.cloneDeep(obj1);
obj1.a = 100;
obj1.b.c = 100;
obj1.b.e.f = 100;
console.log(JSON.stringify(obj1));
{"a":100,"b":{"c":100,"e":{"f":100}}}
console.log(JSON.stringify(obj3));
{"a":0,"b":{"c":0,"e":{"f":0}}}

How can I pass by value a nested Javascript object? [duplicate]

I have an object x. I'd like to copy it as object y, such that changes to y do not modify x. I realized that copying objects derived from built-in JavaScript objects will result in extra, unwanted properties. This isn't a problem, since I'm copying one of my own literal-constructed objects.
How do I correctly clone a JavaScript object?

2022 update
There's a new JS standard called structured cloning. It works in many browsers (see Can I Use).
const clone = structuredClone(object);
Old answer
To do this for any object in JavaScript will not be simple or straightforward. You will run into the problem of erroneously picking up attributes from the object's prototype that should be left in the prototype and not copied to the new instance. If, for instance, you are adding a clone method to Object.prototype, as some answers depict, you will need to explicitly skip that attribute. But what if there are other additional methods added to Object.prototype, or other intermediate prototypes, that you don't know about? In that case, you will copy attributes you shouldn't, so you need to detect unforeseen, non-local attributes with the hasOwnProperty method.
In addition to non-enumerable attributes, you'll encounter a tougher problem when you try to copy objects that have hidden properties. For example, prototype is a hidden property of a function. Also, an object's prototype is referenced with the attribute __proto__, which is also hidden, and will not be copied by a for/in loop iterating over the source object's attributes. I think __proto__ might be specific to Firefox's JavaScript interpreter and it may be something different in other browsers, but you get the picture. Not everything is enumerable. You can copy a hidden attribute if you know its name, but I don't know of any way to discover it automatically.
Yet another snag in the quest for an elegant solution is the problem of setting up the prototype inheritance correctly. If your source object's prototype is Object, then simply creating a new general object with {} will work, but if the source's prototype is some descendant of Object, then you are going to be missing the additional members from that prototype which you skipped using the hasOwnProperty filter, or which were in the prototype, but weren't enumerable in the first place. One solution might be to call the source object's constructor property to get the initial copy object and then copy over the attributes, but then you still will not get non-enumerable attributes. For example, a Date object stores its data as a hidden member:
function clone(obj) {
if (null == obj || "object" != typeof obj) return obj;
var copy = obj.constructor();
for (var attr in obj) {
if (obj.hasOwnProperty(attr)) copy[attr] = obj[attr];
}
return copy;
}
var d1 = new Date();
/* Executes function after 5 seconds. */
setTimeout(function(){
var d2 = clone(d1);
alert("d1 = " + d1.toString() + "\nd2 = " + d2.toString());
}, 5000);
The date string for d1 will be 5 seconds behind that of d2. A way to make one Date the same as another is by calling the setTime method, but that is specific to the Date class. I don't think there is a bullet-proof general solution to this problem, though I would be happy to be wrong!
When I had to implement general deep copying I ended up compromising by assuming that I would only need to copy a plain Object, Array, Date, String, Number, or Boolean. The last 3 types are immutable, so I could perform a shallow copy and not worry about it changing. I further assumed that any elements contained in Object or Array would also be one of the 6 simple types in that list. This can be accomplished with code like the following:
function clone(obj) {
var copy;
// Handle the 3 simple types, and null or undefined
if (null == obj || "object" != typeof obj) return obj;
// Handle Date
if (obj instanceof Date) {
copy = new Date();
copy.setTime(obj.getTime());
return copy;
}
// Handle Array
if (obj instanceof Array) {
copy = [];
for (var i = 0, len = obj.length; i < len; i++) {
copy[i] = clone(obj[i]);
}
return copy;
}
// Handle Object
if (obj instanceof Object) {
copy = {};
for (var attr in obj) {
if (obj.hasOwnProperty(attr)) copy[attr] = clone(obj[attr]);
}
return copy;
}
throw new Error("Unable to copy obj! Its type isn't supported.");
}
The above function will work adequately for the 6 simple types I mentioned, as long as the data in the objects and arrays form a tree structure. That is, there isn't more than one reference to the same data in the object. For example:
// This would be cloneable:
var tree = {
"left" : { "left" : null, "right" : null, "data" : 3 },
"right" : null,
"data" : 8
};
// This would kind-of work, but you would get 2 copies of the
// inner node instead of 2 references to the same copy
var directedAcylicGraph = {
"left" : { "left" : null, "right" : null, "data" : 3 },
"data" : 8
};
directedAcyclicGraph["right"] = directedAcyclicGraph["left"];
// Cloning this would cause a stack overflow due to infinite recursion:
var cyclicGraph = {
"left" : { "left" : null, "right" : null, "data" : 3 },
"data" : 8
};
cyclicGraph["right"] = cyclicGraph;
It will not be able to handle any JavaScript object, but it may be sufficient for many purposes as long as you don't assume that it will just work for anything you throw at it.

If you do not use Dates, functions, undefined, regExp or Infinity within your object, a very simple one liner is JSON.parse(JSON.stringify(object)):
const a = {
string: 'string',
number: 123,
bool: false,
nul: null,
date: new Date(), // stringified
undef: undefined, // lost
inf: Infinity, // forced to 'null'
}
console.log(a);
console.log(typeof a.date); // Date object
const clone = JSON.parse(JSON.stringify(a));
console.log(clone);
console.log(typeof clone.date); // result of .toISOString()
This works for all kind of objects containing objects, arrays, strings, booleans and numbers.
See also this article about the structured clone algorithm of browsers which is used when posting messages to and from a worker. It also contains a function for deep cloning.

In ECMAScript 6 there is Object.assign method, which copies values of all enumerable own properties from one object to another. For example:
var x = {myProp: "value"};
var y = Object.assign({}, x);
But be aware this is a shallow copy - nested objects are still copied as reference.

With jQuery, you can shallow copy with extend:
var copiedObject = jQuery.extend({}, originalObject)
subsequent changes to the copiedObject will not affect the originalObject, and vice versa.
Or to make a deep copy:
var copiedObject = jQuery.extend(true, {}, originalObject)

Per MDN:
If you want shallow copy, use Object.assign({}, a)
For "deep" copy, use JSON.parse(JSON.stringify(a))
There is no need for external libraries but you need to check browser compatibility first.

An elegant way to clone a Javascript object in one line of code
An Object.assign method is part of the ECMAScript 2015 (ES6) standard and does exactly what you need.
var clone = Object.assign({}, obj);
The Object.assign() method is used to copy the values of all enumerable own properties from one or more source objects to a target object.
Read more...
The polyfill to support older browsers:
if (!Object.assign) {
Object.defineProperty(Object, 'assign', {
enumerable: false,
configurable: true,
writable: true,
value: function(target) {
'use strict';
if (target === undefined || target === null) {
throw new TypeError('Cannot convert first argument to object');
}
var to = Object(target);
for (var i = 1; i < arguments.length; i++) {
var nextSource = arguments[i];
if (nextSource === undefined || nextSource === null) {
continue;
}
nextSource = Object(nextSource);
var keysArray = Object.keys(nextSource);
for (var nextIndex = 0, len = keysArray.length; nextIndex < len; nextIndex++) {
var nextKey = keysArray[nextIndex];
var desc = Object.getOwnPropertyDescriptor(nextSource, nextKey);
if (desc !== undefined && desc.enumerable) {
to[nextKey] = nextSource[nextKey];
}
}
}
return to;
}
});
}

There are many answers, but none that mentions Object.create from ECMAScript 5, which admittedly does not give you an exact copy, but sets the source as the prototype of the new object.
Thus, this is not an exact answer to the question, but it is a one-line solution and thus elegant. And it works best for 2 cases:
Where such inheritance is useful (duh!)
Where the source object won't be modified, thus making the relation between the 2 objects a non issue.
Example:
var foo = { a : 1 };
var bar = Object.create(foo);
foo.a; // 1
bar.a; // 1
foo.a = 2;
bar.a; // 2 - prototype changed
bar.a = 3;
foo.a; // Still 2, since setting bar.a makes it an "own" property
Why do I consider this solution to be superior? It's native, thus no looping, no recursion. However, older browsers will need a polyfill.

There are several issues with most solutions on the internet. So I decided to make a follow-up, which includes, why the accepted answer shouldn't be accepted.
starting situation
I want to deep-copy a Javascript Object with all of its children and their children and so on. But since I'm not kind of a normal developer, my Object has normal properties, circular structures and even nested objects.
So let's create a circular structure and a nested object first.
function Circ() {
this.me = this;
}
function Nested(y) {
this.y = y;
}
Let's bring everything together in an Object named a.
var a = {
x: 'a',
circ: new Circ(),
nested: new Nested('a')
};
Next, we want to copy a into a variable named b and mutate it.
var b = a;
b.x = 'b';
b.nested.y = 'b';
You know what happened here because if not you wouldn't even land on this great question.
console.log(a, b);
a --> Object {
x: "b",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
b --> Object {
x: "b",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
Now let's find a solution.
JSON
The first attempt I tried was using JSON.
var b = JSON.parse( JSON.stringify( a ) );
b.x = 'b';
b.nested.y = 'b';
Don't waste too much time on it, you'll get TypeError: Converting circular structure to JSON.
Recursive copy (the accepted "answer")
Let's have a look at the accepted answer.
function cloneSO(obj) {
// Handle the 3 simple types, and null or undefined
if (null == obj || "object" != typeof obj) return obj;
// Handle Date
if (obj instanceof Date) {
var copy = new Date();
copy.setTime(obj.getTime());
return copy;
}
// Handle Array
if (obj instanceof Array) {
var copy = [];
for (var i = 0, len = obj.length; i < len; i++) {
copy[i] = cloneSO(obj[i]);
}
return copy;
}
// Handle Object
if (obj instanceof Object) {
var copy = {};
for (var attr in obj) {
if (obj.hasOwnProperty(attr)) copy[attr] = cloneSO(obj[attr]);
}
return copy;
}
throw new Error("Unable to copy obj! Its type isn't supported.");
}
Looks good, heh? It's a recursive copy of the object and handles other types as well, like Date, but that wasn't a requirement.
var b = cloneSO(a);
b.x = 'b';
b.nested.y = 'b';
Recursion and circular structures doesn't work well together... RangeError: Maximum call stack size exceeded
native solution
After arguing with my co-worker, my boss asked us what happened, and he found a simple solution after some googling. It's called Object.create.
var b = Object.create(a);
b.x = 'b';
b.nested.y = 'b';
This solution was added to Javascript some time ago and even handles circular structure.
console.log(a, b);
a --> Object {
x: "a",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
b --> Object {
x: "b",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
... and you see, it didn't work with the nested structure inside.
polyfill for the native solution
There's a polyfill for Object.create in the older browser just like the IE 8. It's something like recommended by Mozilla, and of course, it's not perfect and results in the same problem as the native solution.
function F() {};
function clonePF(o) {
F.prototype = o;
return new F();
}
var b = clonePF(a);
b.x = 'b';
b.nested.y = 'b';
I've put F outside the scope so we can have a look at what instanceof tells us.
console.log(a, b);
a --> Object {
x: "a",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
b --> F {
x: "b",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
console.log(typeof a, typeof b);
a --> object
b --> object
console.log(a instanceof Object, b instanceof Object);
a --> true
b --> true
console.log(a instanceof F, b instanceof F);
a --> false
b --> true
Same problem as the native solution, but a little bit worse output.
the better (but not perfect) solution
When digging around, I found a similar question (In Javascript, when performing a deep copy, how do I avoid a cycle, due to a property being "this"?) to this one, but with a way better solution.
function cloneDR(o) {
const gdcc = "__getDeepCircularCopy__";
if (o !== Object(o)) {
return o; // primitive value
}
var set = gdcc in o,
cache = o[gdcc],
result;
if (set && typeof cache == "function") {
return cache();
}
// else
o[gdcc] = function() { return result; }; // overwrite
if (o instanceof Array) {
result = [];
for (var i=0; i<o.length; i++) {
result[i] = cloneDR(o[i]);
}
} else {
result = {};
for (var prop in o)
if (prop != gdcc)
result[prop] = cloneDR(o[prop]);
else if (set)
result[prop] = cloneDR(cache);
}
if (set) {
o[gdcc] = cache; // reset
} else {
delete o[gdcc]; // unset again
}
return result;
}
var b = cloneDR(a);
b.x = 'b';
b.nested.y = 'b';
And let's have a look at the output...
console.log(a, b);
a --> Object {
x: "a",
circ: Object {
me: Object { ... }
},
nested: Object {
y: "a"
}
}
b --> Object {
x: "b",
circ: Object {
me: Object { ... }
},
nested: Object {
y: "b"
}
}
console.log(typeof a, typeof b);
a --> object
b --> object
console.log(a instanceof Object, b instanceof Object);
a --> true
b --> true
console.log(a instanceof F, b instanceof F);
a --> false
b --> false
The requirements are matched, but there are still some smaller issues, including changing the instance of nested and circ to Object.
The structure of trees that share a leaf won't be copied, they will become two independent leaves:
[Object] [Object]
/ \ / \
/ \ / \
|/_ _\| |/_ _\|
[Object] [Object] ===> [Object] [Object]
\ / | |
\ / | |
_\| |/_ \|/ \|/
[Object] [Object] [Object]
conclusion
The last solution using recursion and a cache, may not be the best, but it's a real deep-copy of the object. It handles simple properties, circular structures and nested object, but it will mess up the instance of them while cloning.
jsfiddle

If you're okay with a shallow copy, the underscore.js library has a clone method.
y = _.clone(x);
or you can extend it like
copiedObject = _.extend({},originalObject);

OK, imagine you have this object below and you want to clone it:
let obj = {a:1, b:2, c:3}; //ES6
or
var obj = {a:1, b:2, c:3}; //ES5
the answer is mainly depeneds on which ECMAscript you using, in ES6+, you can simply use Object.assign to do the clone:
let cloned = Object.assign({}, obj); //new {a:1, b:2, c:3};
or using spread operator like this:
let cloned = {...obj}; //new {a:1, b:2, c:3};
But if you using ES5, you can use few methods, but the JSON.stringify, just make sure you not using for a big chunk of data to copy, but it could be one line handy way in many cases, something like this:
let cloned = JSON.parse(JSON.stringify(obj));
//new {a:1, b:2, c:3};, can be handy, but avoid using on big chunk of data over and over

Update 06 July 2020
There are three (3) ways to clone objects in JavaScript. As objects in JavaScript are reference values, you can't simply just copy using the =.
The ways are:
const food = { food: 'apple', drink: 'milk' }
// 1. Using the "Spread"
// ------------------
{ ...food }
// 2. Using "Object.assign"
// ------------------
Object.assign({}, food)
// 3. "JSON"
// ------------------
JSON.parse(JSON.stringify(food))
// RESULT:
// { food: 'apple', drink: 'milk' }
This can be used as a reference summary.

One particularly inelegant solution is to use JSON encoding to make deep copies of objects that do not have member methods. The methodology is to JSON encode your target object, then by decoding it, you get the copy you are looking for. You can decode as many times as you want to make as many copies as you need.
Of course, functions do not belong in JSON, so this only works for objects without member methods.
This methodology was perfect for my use case, since I'm storing JSON blobs in a key-value store, and when they are exposed as objects in a JavaScript API, each object actually contains a copy of the original state of the object so we can calculate the delta after the caller has mutated the exposed object.
var object1 = {key:"value"};
var object2 = object1;
object2 = JSON.stringify(object1);
object2 = JSON.parse(object2);
object2.key = "a change";
console.log(object1);// returns value

You can simply use a spread property to copy an object without references. But be careful (see comments), the 'copy' is just on the lowest object/array level. Nested properties are still references!
Complete clone:
let x = {a: 'value1'}
let x2 = {...x}
// => mutate without references:
x2.a = 'value2'
console.log(x.a) // => 'value1'
Clone with references on second level:
const y = {a: {b: 'value3'}}
const y2 = {...y}
// => nested object is still a references:
y2.a.b = 'value4'
console.log(y.a.b) // => 'value4'
JavaScript actually does not support deep clones natively. Use an utility function. For example Ramda:
http://ramdajs.com/docs/#clone

const objClone = { ...obj };
Be aware that nested objects are still copied as a reference.

From this article: How to copy arrays and objects in Javascript by Brian Huisman:
Object.prototype.clone = function() {
var newObj = (this instanceof Array) ? [] : {};
for (var i in this) {
if (i == 'clone') continue;
if (this[i] && typeof this[i] == "object") {
newObj[i] = this[i].clone();
} else newObj[i] = this[i]
} return newObj;
};

For those using AngularJS, there is also direct method for cloning or extending of the objects in this library.
var destination = angular.copy(source);
or
angular.copy(source, destination);
More in angular.copy documentation...

function clone(obj) {
if(obj == null || typeof(obj) != 'object')
return obj;
var temp = new obj.constructor();
for(var key in obj)
temp[key] = clone(obj[key]);
return temp;
}

A.Levy's answer is almost complete, here is my little contribution: there is a way how to handle recursive references, see this line
if(this[attr]==this) copy[attr] = copy;
If the object is XML DOM element, we must use cloneNode instead
if(this.cloneNode) return this.cloneNode(true);
Inspired by A.Levy's exhaustive study and Calvin's prototyping approach, I offer this solution:
Object.prototype.clone = function() {
if(this.cloneNode) return this.cloneNode(true);
var copy = this instanceof Array ? [] : {};
for(var attr in this) {
if(typeof this[attr] == "function" || this[attr]==null || !this[attr].clone)
copy[attr] = this[attr];
else if(this[attr]==this) copy[attr] = copy;
else copy[attr] = this[attr].clone();
}
return copy;
}
Date.prototype.clone = function() {
var copy = new Date();
copy.setTime(this.getTime());
return copy;
}
Number.prototype.clone =
Boolean.prototype.clone =
String.prototype.clone = function() {
return this;
}
See also Andy Burke's note in the answers.

Performance
Today 2020.04.30 I perform tests of chosen solutions on Chrome v81.0, Safari v13.1 and Firefox v75.0 on MacOs High Sierra v10.13.6.
I focus on speed of copy DATA (object with simple type fields, not methods etc.). The solutions A-I can make only shallow copy, solutions J-U can make deep copy.
Results for shallow copy
solution {...obj} (A) is fastest on chrome and firefox and medium fast on safari
solution based on Object.assign (B) is fast on all browsers
jQuery (E) and lodash (F,G,H) solutions are medium/quite fast
solution JSON.parse/stringify (K) is quite slow
solutions D and U are slow on all browsers
Results for deep copy
solution Q is fastest on all browsers
jQuery (L) and lodash (J) are medium fast
solution JSON.parse/stringify (K) is quite slow
solution U is slowest on all browsers
lodash (J) and solution U crash on Chrome for 1000 level deep object
Details
For choosen solutions:
A
B
C(my)
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U,
I perform 4 tests
shallow-small: object with 10 non-nested fields - you can run it HERE
shallow-big: object with 1000 non-nested fields - you can run it HERE
deep-small: object with 10 levels-nested fields - you can run it HERE
deep-big: object with 1000 levels-nested fields - you can run it HERE
Objects used in tests are show in below snippet
let obj_ShallowSmall = {
field0: false,
field1: true,
field2: 1,
field3: 0,
field4: null,
field5: [],
field6: {},
field7: "text7",
field8: "text8",
}
let obj_DeepSmall = {
level0: {
level1: {
level2: {
level3: {
level4: {
level5: {
level6: {
level7: {
level8: {
level9: [[[[[[[[[['abc']]]]]]]]]],
}}}}}}}}},
};
let obj_ShallowBig = Array(1000).fill(0).reduce((a,c,i) => (a['field'+i]=getField(i),a) ,{});
let obj_DeepBig = genDeepObject(1000);
// ------------------
// Show objects
// ------------------
console.log('obj_ShallowSmall:',JSON.stringify(obj_ShallowSmall));
console.log('obj_DeepSmall:',JSON.stringify(obj_DeepSmall));
console.log('obj_ShallowBig:',JSON.stringify(obj_ShallowBig));
console.log('obj_DeepBig:',JSON.stringify(obj_DeepBig));
// ------------------
// HELPERS
// ------------------
function getField(k) {
let i=k%10;
if(i==0) return false;
if(i==1) return true;
if(i==2) return k;
if(i==3) return 0;
if(i==4) return null;
if(i==5) return [];
if(i==6) return {};
if(i>=7) return "text"+k;
}
function genDeepObject(N) {
// generate: {level0:{level1:{...levelN: {end:[[[...N-times...['abc']...]]] }}}...}}}
let obj={};
let o=obj;
let arr = [];
let a=arr;
for(let i=0; i<N; i++) {
o['level'+i]={};
o=o['level'+i];
let aa=[];
a.push(aa);
a=aa;
}
a[0]='abc';
o['end']=arr;
return obj;
}
Below snippet presents tested solutions and shows differences between them
function A(obj) {
return {...obj}
}
function B(obj) {
return Object.assign({}, obj);
}
function C(obj) {
return Object.keys(obj).reduce( (a,c) => (a[c]=obj[c], a), {})
}
function D(obj) {
let copyOfObject = {};
Object.defineProperties(copyOfObject, Object.getOwnPropertyDescriptors(obj));
return copyOfObject;
}
function E(obj) {
return jQuery.extend({}, obj) // shallow
}
function F(obj) {
return _.clone(obj);
}
function G(obj) {
return _.clone(obj,true);
}
function H(obj) {
return _.extend({},obj);
}
function I(obj) {
if (null == obj || "object" != typeof obj) return obj;
var copy = obj.constructor();
for (var attr in obj) {
if (obj.hasOwnProperty(attr)) copy[attr] = obj[attr];
}
return copy;
}
function J(obj) {
return _.cloneDeep(obj,true);
}
function K(obj) {
return JSON.parse(JSON.stringify(obj));
}
function L(obj) {
return jQuery.extend(true, {}, obj) // deep
}
function M(obj) {
if(obj == null || typeof(obj) != 'object')
return obj;
var temp = new obj.constructor();
for(var key in obj)
temp[key] = M(obj[key]);
return temp;
}
function N(obj) {
let EClone = function(obj) {
var newObj = (obj instanceof Array) ? [] : {};
for (var i in obj) {
if (i == 'EClone') continue;
if (obj[i] && typeof obj[i] == "object") {
newObj[i] = EClone(obj[i]);
} else newObj[i] = obj[i]
} return newObj;
};
return EClone(obj);
};
function O(obj) {
if (obj == null || typeof obj != "object") return obj;
if (obj.constructor != Object && obj.constructor != Array) return obj;
if (obj.constructor == Date || obj.constructor == RegExp || obj.constructor == Function ||
obj.constructor == String || obj.constructor == Number || obj.constructor == Boolean)
return new obj.constructor(obj);
let to = new obj.constructor();
for (var name in obj)
{
to[name] = typeof to[name] == "undefined" ? O(obj[name], null) : to[name];
}
return to;
}
function P(obj) {
function clone(target, source){
for(let key in source){
// Use getOwnPropertyDescriptor instead of source[key] to prevent from trigering setter/getter.
let descriptor = Object.getOwnPropertyDescriptor(source, key);
if(descriptor.value instanceof String){
target[key] = new String(descriptor.value);
}
else if(descriptor.value instanceof Array){
target[key] = clone([], descriptor.value);
}
else if(descriptor.value instanceof Object){
let prototype = Reflect.getPrototypeOf(descriptor.value);
let cloneObject = clone({}, descriptor.value);
Reflect.setPrototypeOf(cloneObject, prototype);
target[key] = cloneObject;
}
else {
Object.defineProperty(target, key, descriptor);
}
}
let prototype = Reflect.getPrototypeOf(source);
Reflect.setPrototypeOf(target, prototype);
return target;
}
return clone({},obj);
}
function Q(obj) {
var copy;
// Handle the 3 simple types, and null or undefined
if (null == obj || "object" != typeof obj) return obj;
// Handle Date
if (obj instanceof Date) {
copy = new Date();
copy.setTime(obj.getTime());
return copy;
}
// Handle Array
if (obj instanceof Array) {
copy = [];
for (var i = 0, len = obj.length; i < len; i++) {
copy[i] = Q(obj[i]);
}
return copy;
}
// Handle Object
if (obj instanceof Object) {
copy = {};
for (var attr in obj) {
if (obj.hasOwnProperty(attr)) copy[attr] = Q(obj[attr]);
}
return copy;
}
throw new Error("Unable to copy obj! Its type isn't supported.");
}
function R(obj) {
const gdcc = "__getDeepCircularCopy__";
if (obj !== Object(obj)) {
return obj; // primitive value
}
var set = gdcc in obj,
cache = obj[gdcc],
result;
if (set && typeof cache == "function") {
return cache();
}
// else
obj[gdcc] = function() { return result; }; // overwrite
if (obj instanceof Array) {
result = [];
for (var i=0; i<obj.length; i++) {
result[i] = R(obj[i]);
}
} else {
result = {};
for (var prop in obj)
if (prop != gdcc)
result[prop] = R(obj[prop]);
else if (set)
result[prop] = R(cache);
}
if (set) {
obj[gdcc] = cache; // reset
} else {
delete obj[gdcc]; // unset again
}
return result;
}
function S(obj) {
const cache = new WeakMap(); // Map of old - new references
function copy(object) {
if (typeof object !== 'object' ||
object === null ||
object instanceof HTMLElement
)
return object; // primitive value or HTMLElement
if (object instanceof Date)
return new Date().setTime(object.getTime());
if (object instanceof RegExp)
return new RegExp(object.source, object.flags);
if (cache.has(object))
return cache.get(object);
const result = object instanceof Array ? [] : {};
cache.set(object, result); // store reference to object before the recursive starts
if (object instanceof Array) {
for(const o of object) {
result.push(copy(o));
}
return result;
}
const keys = Object.keys(object);
for (const key of keys)
result[key] = copy(object[key]);
return result;
}
return copy(obj);
}
function T(obj){
var clonedObjectsArray = [];
var originalObjectsArray = []; //used to remove the unique ids when finished
var next_objid = 0;
function objectId(obj) {
if (obj == null) return null;
if (obj.__obj_id == undefined){
obj.__obj_id = next_objid++;
originalObjectsArray[obj.__obj_id] = obj;
}
return obj.__obj_id;
}
function cloneRecursive(obj) {
if (null == obj || typeof obj == "string" || typeof obj == "number" || typeof obj == "boolean") return obj;
// Handle Date
if (obj instanceof Date) {
var copy = new Date();
copy.setTime(obj.getTime());
return copy;
}
// Handle Array
if (obj instanceof Array) {
var copy = [];
for (var i = 0; i < obj.length; ++i) {
copy[i] = cloneRecursive(obj[i]);
}
return copy;
}
// Handle Object
if (obj instanceof Object) {
if (clonedObjectsArray[objectId(obj)] != undefined)
return clonedObjectsArray[objectId(obj)];
var copy;
if (obj instanceof Function)//Handle Function
copy = function(){return obj.apply(this, arguments);};
else
copy = {};
clonedObjectsArray[objectId(obj)] = copy;
for (var attr in obj)
if (attr != "__obj_id" && obj.hasOwnProperty(attr))
copy[attr] = cloneRecursive(obj[attr]);
return copy;
}
throw new Error("Unable to copy obj! Its type isn't supported.");
}
var cloneObj = cloneRecursive(obj);
//remove the unique ids
for (var i = 0; i < originalObjectsArray.length; i++)
{
delete originalObjectsArray[i].__obj_id;
};
return cloneObj;
}
function U(obj) {
/*
Deep copy objects by value rather than by reference,
exception: `Proxy`
*/
const seen = new WeakMap()
return clone(obj)
function defineProp(object, key, descriptor = {}, copyFrom = {}) {
const { configurable: _configurable, writable: _writable }
= Object.getOwnPropertyDescriptor(object, key)
|| { configurable: true, writable: true }
const test = _configurable // Can redefine property
&& (_writable === undefined || _writable) // Can assign to property
if (!test || arguments.length <= 2) return test
const basisDesc = Object.getOwnPropertyDescriptor(copyFrom, key)
|| { configurable: true, writable: true } // Custom…
|| {}; // …or left to native default settings
["get", "set", "value", "writable", "enumerable", "configurable"]
.forEach(attr =>
descriptor[attr] === undefined &&
(descriptor[attr] = basisDesc[attr])
)
const { get, set, value, writable, enumerable, configurable }
= descriptor
return Object.defineProperty(object, key, {
enumerable, configurable, ...get || set
? { get, set } // Accessor descriptor
: { value, writable } // Data descriptor
})
}
function clone(object) {
if (object !== Object(object)) return object /*
—— Check if the object belongs to a primitive data type */
if (object instanceof Node) return object.cloneNode(true) /*
—— Clone DOM trees */
let _object // The clone of object
switch (object.constructor) {
case Array:
case Object:
_object = cloneObject(object)
break
case Date:
_object = new Date(+object)
break
case Function:
const fnStr = String(object)
_object = new Function("return " +
(/^(?!function |[^{]+?=>)[^(]+?\(/.test(fnStr)
? "function " : ""
) + fnStr
)()
copyPropDescs(_object, object)
break
case RegExp:
_object = new RegExp(object)
break
default:
switch (Object.prototype.toString.call(object.constructor)) {
// // Stem from:
case "[object Function]": // `class`
case "[object Undefined]": // `Object.create(null)`
_object = cloneObject(object)
break
default: // `Proxy`
_object = object
}
}
return _object
}
function cloneObject(object) {
if (seen.has(object)) return seen.get(object) /*
—— Handle recursive references (circular structures) */
const _object = Array.isArray(object)
? []
: Object.create(Object.getPrototypeOf(object)) /*
—— Assign [[Prototype]] for inheritance */
seen.set(object, _object) /*
—— Make `_object` the associative mirror of `object` */
Reflect.ownKeys(object).forEach(key =>
defineProp(_object, key, { value: clone(object[key]) }, object)
)
return _object
}
function copyPropDescs(target, source) {
Object.defineProperties(target,
Object.getOwnPropertyDescriptors(source)
)
}
}
// ------------------------
// Test properties
// ------------------------
console.log(` shallow deep func circ undefined date RegExp bigInt`)
log(A);
log(B);
log(C);
log(D);
log(E);
log(F);
log(G);
log(H);
log(I);
log(J);
log(K);
log(L);
log(M);
log(N);
log(O);
log(P);
log(Q);
log(R);
log(S);
log(T);
log(U);
console.log(` shallow deep func circ undefined date RegExp bigInt
----
LEGEND:
shallow - solution create shallow copy
deep - solution create deep copy
func - solution copy functions
circ - solution can copy object with circular references
undefined - solution copy fields with undefined value
date - solution can copy date
RegExp - solution can copy fields with regular expressions
bigInt - solution can copy BigInt
`)
// ------------------------
// Helper functions
// ------------------------
function deepCompare(obj1,obj2) {
return JSON.stringify(obj1)===JSON.stringify(obj2);
}
function getCase() { // pure data case
return {
undef: undefined,
bool: true, num: 1, str: "txt1",
e1: null, e2: [], e3: {}, e4: 0, e5: false,
arr: [ false, 2, "txt3", null, [], {},
[ true,4,"txt5",null, [], {}, [true,6,"txt7",null,[],{} ],
{bool: true,num: 8, str: "txt9", e1:null, e2:[] ,e3:{} ,e4: 0, e5: false}
],
{bool: true,num: 10, str: "txt11", e1:null, e2:[] ,e3:{} ,e4: 0, e5: false}
],
obj: {
bool: true, num: 12, str: "txt13",
e1: null, e2: [], e3: {}, e4: 0, e5: false,
arr: [true,14,"txt15",null,[],{} ],
obj: {
bool: true, num: 16, str: "txt17",
e1: null, e2: [], e3: {}, e4: 0, e5: false,
arr: [true,18,"txt19",null,[],{} ],
obj: {bool: true,num: 20, str: "txt21", e1:null, e2:[] ,e3:{} ,e4: 0, e5: false}
}
}
};
}
function check(org, copy, field, newValue) {
copy[field] = newValue;
return deepCompare(org,copy);
}
function testFunc(f) {
let o = { a:1, fun: (i,j)=> i+j };
let c = f(o);
let val = false
try{
val = c.fun(3,4)==7;
} catch(e) { }
return val;
}
function testCirc(f) {
function Circ() {
this.me = this;
}
var o = {
x: 'a',
circ: new Circ(),
obj_circ: null,
};
o.obj_circ = o;
let val = false;
try{
let c = f(o);
val = (o.obj_circ == o) && (o.circ == o.circ.me);
} catch(e) { }
return val;
}
function testRegExp(f) {
let o = {
re: /a[0-9]+/,
};
let val = false;
try{
let c = f(o);
val = (String(c.re) == String(/a[0-9]+/));
} catch(e) { }
return val;
}
function testDate(f) {
let o = {
date: new Date(),
};
let val = false;
try{
let c = f(o);
val = (+new Date(c.date) == +new Date(o.date));
} catch(e) { }
return val;
}
function testBigInt(f) {
let val = false;
try{
let o = {
big: 123n,
};
let c = f(o);
val = o.big == c.big;
} catch(e) { }
return val;
}
function log(f) {
let o = getCase(); // orginal object
let oB = getCase(); // "backup" used for shallow valid test
let c1 = f(o); // copy 1 for reference
let c2 = f(o); // copy 2 for test shallow values
let c3 = f(o); // copy 3 for test deep values
let is_proper_copy = deepCompare(c1,o); // shoud be true
// shallow changes
let testShallow =
[ ['bool',false],['num',666],['str','xyz'],['arr',[]],['obj',{}] ]
.reduce((acc,curr)=> acc && check(c1,c2,curr[0], curr[1]), true );
// should be true (original object shoud not have changed shallow fields)
let is_valid = deepCompare(o,oB);
// deep test (intruduce some change)
if (c3.arr[6]) c3.arr[6][7].num = 777;
let diff_shallow = !testShallow; // shoud be true (shallow field was copied)
let diff_deep = !deepCompare(c1,c3); // shoud be true (deep field was copied)
let can_copy_functions = testFunc(f);
let can_copy_circular = testCirc(f);
let can_copy_regexp = testRegExp(f);
let can_copy_date = testDate(f);
let can_copy_bigInt = testBigInt(f);
let has_undefined = 'undef' in c1; // field with undefined value is copied?
let is_ok = is_valid && is_proper_copy;
let b=(bool) => (bool+'').padEnd(5,' '); // bool value to formated string
testFunc(f);
if(is_ok) {
console.log(`${f.name} ${b(diff_shallow)} ${b(diff_deep)} ${b(can_copy_functions)} ${b(can_copy_circular)} ${b(has_undefined)} ${b(can_copy_date)} ${b(can_copy_regexp)} ${b(can_copy_bigInt)}`)
} else {
console.log(`${f.name}: INVALID ${is_valid} ${is_proper_copy}`,{c1})
}
}
<script src="https://code.jquery.com/jquery-3.5.0.min.js" integrity="sha256-xNzN2a4ltkB44Mc/Jz3pT4iU1cmeR0FkXs4pru/JxaQ=" crossorigin="anonymous"></script>
<script src="https://cdn.jsdelivr.net/npm/lodash#4.17.15/lodash.min.js"></script>
This snippet only presents tested solutions and show differences between them (but it no make performence tests)
Below there are example results for Chrome for shallow-big object

Using Lodash:
var y = _.clone(x, true);

In ES-6 you can simply use Object.assign(...).
Ex:
let obj = {person: 'Thor Odinson'};
let clone = Object.assign({}, obj);
A good reference is here:
https://googlechrome.github.io/samples/object-assign-es6/

Interested in cloning simple objects:
JSON.parse(JSON.stringify(json_original));
Source : How to copy JavaScript object to new variable NOT by reference?

let clone = Object.assign( Object.create( Object.getPrototypeOf(obj)), obj)
ES6 solution if you want to (shallow) clone a class instance and not just a property object.

Structured Cloning
2022 update: The structuredClone() global function is already available in Node 17, Deno 1.14, and most major browsers (see Can I Use).
You can use the same structured clone mechanism that the HTML standard includes for sending data between realms.
const clone = structuredClone(original);
See the other answer for more details.

You can clone an object and remove any reference from the previous one using a single line of code. Simply do:
var obj1 = { text: 'moo1' };
var obj2 = Object.create(obj1); // Creates a new clone without references
obj2.text = 'moo2'; // Only updates obj2's text property
console.log(obj1, obj2); // Outputs: obj1: {text:'moo1'}, obj2: {text:'moo2'}
For browsers / engines that do not currently support Object.create you can use this polyfill:
// Polyfill Object.create if it does not exist
if (!Object.create) {
Object.create = function (o) {
var F = function () {};
F.prototype = o;
return new F();
};
}

New answer to an old question! If you have the pleasure of having using ECMAScript 2016 (ES6) with Spread Syntax, it's easy.
keepMeTheSame = {first: "Me!", second: "You!"};
cloned = {...keepMeTheSame}
This provides a clean method for a shallow copy of an object. Making a deep copy, meaning makign a new copy of every value in every recursively nested object, requires on of the heavier solutions above.
JavaScript keeps evolving.

I think there is a simple and working answer. In deep copying there are two concerns:
Keep properties independent to each other.
And keep the methods alive on cloned object.
So I think one simple solution will be to first serialize and deserialize and then do an assign on it to copy functions too.
let deepCloned = JSON.parse(JSON.stringify(source));
let merged = Object.assign({}, source);
Object.assign(merged, deepCloned);
Although this question has many answers, I hope this one helps too.

For a deep copy and clone, JSON.stringify then JSON.parse the object:
obj = { a: 0 , b: { c: 0}};
let deepClone = JSON.parse(JSON.stringify(obj));
obj.a = 5;
obj.b.c = 5;
console.log(JSON.stringify(deepClone)); // { a: 0, b: { c: 0}}

(The following was mainly an integration of #Maciej Bukowski, #A. Levy, #Jan Turoň, #Redu's answers, and #LeviRoberts, #RobG's comments, many thanks to them!!!)
Deep copy? — YES! (mostly);
Shallow copy? — NO! (except Proxy).
I sincerely welcome everyone to test clone().
In addition, defineProp() is designed to easily and quickly (re)define or copy any type of descriptor.
Function
function clone(object) {
/*
Deep copy objects by value rather than by reference,
exception: `Proxy`
*/
const seen = new WeakMap()
return clone(object)
function clone(object) {
if (object !== Object(object)) return object /*
—— Check if the object belongs to a primitive data type */
if (object instanceof Node) return object.cloneNode(true) /*
—— Clone DOM trees */
let _object // The clone of object
switch (object.constructor) {
case Array:
case Object:
_object = cloneObject(object)
break
case Date:
_object = new Date(+object)
break
case Function:
_object = copyFn(object)
break
case RegExp:
_object = new RegExp(object)
break
default:
switch (Object.prototype.toString.call(object.constructor)) {
// // Stem from:
case "[object Function]":
switch (object[Symbol.toStringTag]) {
case undefined:
_object = cloneObject(object) // `class`
break
case "AsyncFunction":
case "GeneratorFunction":
case "AsyncGeneratorFunction":
_object = copyFn(object)
break
default:
_object = object
}
break
case "[object Undefined]": // `Object.create(null)`
_object = cloneObject(object)
break
default:
_object = object // `Proxy`
}
}
return _object
}
function cloneObject(object) {
if (seen.has(object)) return seen.get(object) /*
—— Handle recursive references (circular structures) */
const _object = Array.isArray(object)
? []
: Object.create(Object.getPrototypeOf(object)) /*
—— Assign [[Prototype]] for inheritance */
seen.set(object, _object) /*
—— Make `_object` the associative mirror of `object` */
Reflect.ownKeys(object).forEach(key =>
defineProp(_object, key, { value: clone(object[key]) }, object)
)
return _object
}
}
function copyPropDescs(target, source) {
Object.defineProperties(target,
Object.getOwnPropertyDescriptors(source)
)
}
function convertFnToStr(fn) {
let fnStr = String(fn)
if (fn.name.startsWith("[")) // isSymbolKey
fnStr = fnStr.replace(/\[Symbol\..+?\]/, '')
fnStr = /^(?!(async )?(function\b|[^{]+?=>))[^(]+?\(/.test(fnStr)
? fnStr.replace(/^(async )?(\*)?/, "$1function$2 ") : fnStr
return fnStr
}
function copyFn(fn) {
const newFn = new Function(`return ${convertFnToStr(fn)}`)()
copyPropDescs(newFn, fn)
return newFn
}
function defineProp(object, key, descriptor = {}, copyFrom = {}) {
const { configurable: _configurable, writable: _writable }
= Object.getOwnPropertyDescriptor(object, key)
|| { configurable: true, writable: true }
const test = _configurable // Can redefine property
&& (_writable === undefined || _writable) // Can assign to property
if (!test || arguments.length <= 2) return test
const basisDesc = Object.getOwnPropertyDescriptor(copyFrom, key)
|| { configurable: true, writable: true } // Custom…
|| {}; // …or left to native default settings
["get", "set", "value", "writable", "enumerable", "configurable"]
.forEach(attr =>
descriptor[attr] === undefined &&
(descriptor[attr] = basisDesc[attr])
)
const { get, set, value, writable, enumerable, configurable }
= descriptor
return Object.defineProperty(object, key, {
enumerable, configurable, ...get || set
? { get, set } // Accessor descriptor
: { value, writable } // Data descriptor
})
}
// Tests
const obj0 = {
u: undefined,
nul: null,
t: true,
num: 9,
str: "",
sym: Symbol("symbol"),
[Symbol("e")]: Math.E,
arr: [[0], [1, 2]],
d: new Date(),
re: /f/g,
get g() { return 0 },
o: {
n: 0,
o: { f: function (...args) { } }
},
f: {
getAccessorStr(object) {
return []
.concat(...
Object.values(Object.getOwnPropertyDescriptors(object))
.filter(desc => desc.writable === undefined)
.map(desc => Object.values(desc))
)
.filter(prop => typeof prop === "function")
.map(String)
},
f0: function f0() { },
f1: function () { },
f2: a => a / (a + 1),
f3: () => 0,
f4(params) { return param => param + params },
f5: (a, b) => ({ c = 0 } = {}) => a + b + c
}
}
defineProp(obj0, "s", { set(v) { this._s = v } })
defineProp(obj0.arr, "tint", { value: { is: "non-enumerable" } })
obj0.arr[0].name = "nested array"
let obj1 = clone(obj0)
obj1.o.n = 1
obj1.o.o.g = function g(a = 0, b = 0) { return a + b }
obj1.arr[1][1] = 3
obj1.d.setTime(+obj0.d + 60 * 1000)
obj1.arr.tint.is = "enumerable? no"
obj1.arr[0].name = "a nested arr"
defineProp(obj1, "s", { set(v) { this._s = v + 1 } })
defineProp(obj1.re, "multiline", { value: true })
console.log("\n\n" + "-".repeat(2 ** 6))
console.log(">:>: Test - Routinely")
console.log("obj0:\n ", JSON.stringify(obj0))
console.log("obj1:\n ", JSON.stringify(obj1))
console.log()
console.log("obj0:\n ", obj0)
console.log("obj1:\n ", obj1)
console.log()
console.log("obj0\n ",
".arr.tint:", obj0.arr.tint, "\n ",
".arr[0].name:", obj0.arr[0].name
)
console.log("obj1\n ",
".arr.tint:", obj1.arr.tint, "\n ",
".arr[0].name:", obj1.arr[0].name
)
console.log()
console.log("Accessor-type descriptor\n ",
"of obj0:", obj0.f.getAccessorStr(obj0), "\n ",
"of obj1:", obj1.f.getAccessorStr(obj1), "\n ",
"set (obj0 & obj1) .s :", obj0.s = obj1.s = 0, "\n ",
" → (obj0 , obj1) ._s:", obj0._s, ",", obj1._s
)
console.log("—— obj0 has not been interfered.")
console.log("\n\n" + "-".repeat(2 ** 6))
console.log(">:>: Test - More kinds of functions")
const fnsForTest = {
f(_) { return _ },
func: _ => _,
aFunc: async _ => _,
async function() { },
async asyncFunc() { },
aFn: async function () { },
*gen() { },
async *asyncGen() { },
aG1: async function* () { },
aG2: async function* gen() { },
*[Symbol.iterator]() { yield* Object.keys(this) }
}
console.log(Reflect.ownKeys(fnsForTest).map(k =>
`${String(k)}:
${fnsForTest[k].name}-->
${String(fnsForTest[k])}`
).join("\n"))
const normedFnsStr = `{
f: function f(_) { return _ },
func: _ => _,
aFunc: async _ => _,
function: async function() { },
asyncFunc: async function asyncFunc() { },
aFn: async function () { },
gen: function* gen() { },
asyncGen: async function* asyncGen() { },
aG1: async function* () { },
aG2: async function* gen() { },
[Symbol.iterator]: function* () { yield* Object.keys(this) }
}`
const copiedFnsForTest = clone(fnsForTest)
console.log("fnsForTest:", fnsForTest)
console.log("fnsForTest (copied):", copiedFnsForTest)
console.log("fnsForTest (normed str):", eval(`(${normedFnsStr})`))
console.log("Comparison of fnsForTest and its clone:",
Reflect.ownKeys(fnsForTest).map(k =>
[k, fnsForTest[k] === copiedFnsForTest[k]]
)
)
console.log("\n\n" + "-".repeat(2 ** 6))
console.log(">:>: Test - Circular structures")
obj0.o.r = {}
obj0.o.r.recursion = obj0.o
obj0.arr[1] = obj0.arr
obj1 = clone(obj0)
console.log("obj0:\n ", obj0)
console.log("obj1:\n ", obj1)
console.log("Clear obj0's recursion:",
obj0.o.r.recursion = null, obj0.arr[1] = 1
)
console.log(
"obj0\n ",
".o.r:", obj0.o.r, "\n ",
".arr:", obj0.arr
)
console.log(
"obj1\n ",
".o.r:", obj1.o.r, "\n ",
".arr:", obj1.arr
)
console.log("—— obj1 has not been interfered.")
console.log("\n\n" + "-".repeat(2 ** 6))
console.log(">:>: Test - Classes")
class Person {
constructor(name) {
this.name = name
}
}
class Boy extends Person { }
Boy.prototype.sex = "M"
const boy0 = new Boy
boy0.hobby = { sport: "spaceflight" }
const boy1 = clone(boy0)
boy1.hobby.sport = "superluminal flight"
boy0.name = "one"
boy1.name = "neo"
console.log("boy0:\n ", boy0)
console.log("boy1:\n ", boy1)
console.log("boy1's prototype === boy0's:",
Object.getPrototypeOf(boy1) === Object.getPrototypeOf(boy0)
)
References
Object.create() | MDN
Object.defineProperties() | MDN
Enumerability and ownership of properties | MDN
TypeError: cyclic object value | MDN
Language tricks used
Conditionally add prop to object

Use lodash _.cloneDeep().
Shallow Copy: lodash _.clone()
A shallow copy can be made by simply copying the reference.
let obj1 = {
a: 0,
b: {
c: 0,
e: {
f: 0
}
}
};
let obj3 = _.clone(obj1);
obj1.a = 4;
obj1.b.c = 4;
obj1.b.e.f = 100;
console.log(JSON.stringify(obj1));
//{"a":4,"b":{"c":4,"e":{"f":100}}}
console.log(JSON.stringify(obj3));
//{"a":0,"b":{"c":4,"e":{"f":100}}}
Deep Copy: lodash _.cloneDeep()
fields are dereferenced: rather than references to objects being copied
let obj1 = {
a: 0,
b: {
c: 0,
e: {
f: 0
}
}
};
let obj3 = _.cloneDeep(obj1);
obj1.a = 100;
obj1.b.c = 100;
obj1.b.e.f = 100;
console.log(JSON.stringify(obj1));
{"a":100,"b":{"c":100,"e":{"f":100}}}
console.log(JSON.stringify(obj3));
{"a":0,"b":{"c":0,"e":{"f":0}}}

Removing properties of an object starting by a parameter

I would like some advice to return the properties of an object where others are deleted using lodash.
Example :
deletePropsThatStarsWith({a:1, b:2, aa:3}, 'a') //ouputs {b:2}
So far this is what is working but i don't know where i need to go.
var obj = { a: 1, b: 2, aa: 3 }
function deletePropsThatStartsWith(object, prop) {
return _.omit(object, prop)
}
console.log(deletePropsThatStartsWith(obj, 'a'))
<script src="https://cdn.jsdelivr.net/npm/lodash#4.17.10/lodash.min.js"></script>
Thanks for help/advices.

You're looking for the lodash _.omitBy method. You can pass a callback for the second parameter, which decides which properties to omit. The callback will be invoked with two arguments, value and key.
So for your example, all you need is to check the key, and see if it starts with the desired string. The .startsWith method will be a good choice, or if you can't use that, lodash itself provides a _.startsWith method.
Example:
var obj = {a: 1, b: 2, aa: 3}
function deletePropsThatStartsWith(object, prop) {
return _.omitBy(object, (v, k) => k.startsWith(prop))
}
console.log(deletePropsThatStartsWith(obj, 'a'))
<script src="https://cdn.jsdelivr.net/npm/lodash#4.17.10/lodash.min.js"></script>

You don't need lodash for this, try to implement it by yourself:
function deletePropsThatStartsWith(object, letter) {
var newObjc = {}
Object.keys(object).forEach((key) => {
if (!key.startsWith(letter)) {
newObjc = { ...newObjc, [key]: object[key] }
}
})
return newObjc
}
const objc = { a: 1, b: 2, aa: 3 }
console.log(deletePropsThatStartsWith(objc, 'a'))
The output is { b: 2 }

Alternatively, you can get the object keys and not use lodash at all. This is built into vanilla js (my favorite JS framework!)
function deletePropsThatStartsWith(oldObject, prop) {
let newObject = {};
Object.keys(oldObject).forEach(k => {
if (k.startsWith(prop)) continue;
newObject[k] = oldObject[k];
});
return newObject;
};

// without lodash
function deletePropsThatStartsWith(obj, prop) {
var keys = Object.keys(obj);
var newObj = {};
for(var i = 0;i < keys.length;i++){
if(keys[i][0] !== prop){
newObj[keys[i]] = obj[keys[i]]
}
}
return newObj;
}
console.log(deletePropsThatStartsWith({a:1,b:2,aa:3},'a'));

You can do this with a simple for..in loop:
function deletePropsThatStartsWith(object, prop) {
let copy = {};
for (key in object) {
if (!key.startsWith(prop))
copy[key] = object[key]
};
return copy;
}
var obj = { a: 1, b: 2, aa: 3 }
console.log(deletePropsThatStartsWith(obj, "a"));

javascript: Strange behavior when setting null for value of one object [duplicate]

I have an object x. I'd like to copy it as object y, such that changes to y do not modify x. I realized that copying objects derived from built-in JavaScript objects will result in extra, unwanted properties. This isn't a problem, since I'm copying one of my own literal-constructed objects.
How do I correctly clone a JavaScript object?

2022 update
There's a new JS standard called structured cloning. It works in many browsers (see Can I Use).
const clone = structuredClone(object);
Old answer
To do this for any object in JavaScript will not be simple or straightforward. You will run into the problem of erroneously picking up attributes from the object's prototype that should be left in the prototype and not copied to the new instance. If, for instance, you are adding a clone method to Object.prototype, as some answers depict, you will need to explicitly skip that attribute. But what if there are other additional methods added to Object.prototype, or other intermediate prototypes, that you don't know about? In that case, you will copy attributes you shouldn't, so you need to detect unforeseen, non-local attributes with the hasOwnProperty method.
In addition to non-enumerable attributes, you'll encounter a tougher problem when you try to copy objects that have hidden properties. For example, prototype is a hidden property of a function. Also, an object's prototype is referenced with the attribute __proto__, which is also hidden, and will not be copied by a for/in loop iterating over the source object's attributes. I think __proto__ might be specific to Firefox's JavaScript interpreter and it may be something different in other browsers, but you get the picture. Not everything is enumerable. You can copy a hidden attribute if you know its name, but I don't know of any way to discover it automatically.
Yet another snag in the quest for an elegant solution is the problem of setting up the prototype inheritance correctly. If your source object's prototype is Object, then simply creating a new general object with {} will work, but if the source's prototype is some descendant of Object, then you are going to be missing the additional members from that prototype which you skipped using the hasOwnProperty filter, or which were in the prototype, but weren't enumerable in the first place. One solution might be to call the source object's constructor property to get the initial copy object and then copy over the attributes, but then you still will not get non-enumerable attributes. For example, a Date object stores its data as a hidden member:
function clone(obj) {
if (null == obj || "object" != typeof obj) return obj;
var copy = obj.constructor();
for (var attr in obj) {
if (obj.hasOwnProperty(attr)) copy[attr] = obj[attr];
}
return copy;
}
var d1 = new Date();
/* Executes function after 5 seconds. */
setTimeout(function(){
var d2 = clone(d1);
alert("d1 = " + d1.toString() + "\nd2 = " + d2.toString());
}, 5000);
The date string for d1 will be 5 seconds behind that of d2. A way to make one Date the same as another is by calling the setTime method, but that is specific to the Date class. I don't think there is a bullet-proof general solution to this problem, though I would be happy to be wrong!
When I had to implement general deep copying I ended up compromising by assuming that I would only need to copy a plain Object, Array, Date, String, Number, or Boolean. The last 3 types are immutable, so I could perform a shallow copy and not worry about it changing. I further assumed that any elements contained in Object or Array would also be one of the 6 simple types in that list. This can be accomplished with code like the following:
function clone(obj) {
var copy;
// Handle the 3 simple types, and null or undefined
if (null == obj || "object" != typeof obj) return obj;
// Handle Date
if (obj instanceof Date) {
copy = new Date();
copy.setTime(obj.getTime());
return copy;
}
// Handle Array
if (obj instanceof Array) {
copy = [];
for (var i = 0, len = obj.length; i < len; i++) {
copy[i] = clone(obj[i]);
}
return copy;
}
// Handle Object
if (obj instanceof Object) {
copy = {};
for (var attr in obj) {
if (obj.hasOwnProperty(attr)) copy[attr] = clone(obj[attr]);
}
return copy;
}
throw new Error("Unable to copy obj! Its type isn't supported.");
}
The above function will work adequately for the 6 simple types I mentioned, as long as the data in the objects and arrays form a tree structure. That is, there isn't more than one reference to the same data in the object. For example:
// This would be cloneable:
var tree = {
"left" : { "left" : null, "right" : null, "data" : 3 },
"right" : null,
"data" : 8
};
// This would kind-of work, but you would get 2 copies of the
// inner node instead of 2 references to the same copy
var directedAcylicGraph = {
"left" : { "left" : null, "right" : null, "data" : 3 },
"data" : 8
};
directedAcyclicGraph["right"] = directedAcyclicGraph["left"];
// Cloning this would cause a stack overflow due to infinite recursion:
var cyclicGraph = {
"left" : { "left" : null, "right" : null, "data" : 3 },
"data" : 8
};
cyclicGraph["right"] = cyclicGraph;
It will not be able to handle any JavaScript object, but it may be sufficient for many purposes as long as you don't assume that it will just work for anything you throw at it.

If you do not use Dates, functions, undefined, regExp or Infinity within your object, a very simple one liner is JSON.parse(JSON.stringify(object)):
const a = {
string: 'string',
number: 123,
bool: false,
nul: null,
date: new Date(), // stringified
undef: undefined, // lost
inf: Infinity, // forced to 'null'
}
console.log(a);
console.log(typeof a.date); // Date object
const clone = JSON.parse(JSON.stringify(a));
console.log(clone);
console.log(typeof clone.date); // result of .toISOString()
This works for all kind of objects containing objects, arrays, strings, booleans and numbers.
See also this article about the structured clone algorithm of browsers which is used when posting messages to and from a worker. It also contains a function for deep cloning.

In ECMAScript 6 there is Object.assign method, which copies values of all enumerable own properties from one object to another. For example:
var x = {myProp: "value"};
var y = Object.assign({}, x);
But be aware this is a shallow copy - nested objects are still copied as reference.

With jQuery, you can shallow copy with extend:
var copiedObject = jQuery.extend({}, originalObject)
subsequent changes to the copiedObject will not affect the originalObject, and vice versa.
Or to make a deep copy:
var copiedObject = jQuery.extend(true, {}, originalObject)

Per MDN:
If you want shallow copy, use Object.assign({}, a)
For "deep" copy, use JSON.parse(JSON.stringify(a))
There is no need for external libraries but you need to check browser compatibility first.

An elegant way to clone a Javascript object in one line of code
An Object.assign method is part of the ECMAScript 2015 (ES6) standard and does exactly what you need.
var clone = Object.assign({}, obj);
The Object.assign() method is used to copy the values of all enumerable own properties from one or more source objects to a target object.
Read more...
The polyfill to support older browsers:
if (!Object.assign) {
Object.defineProperty(Object, 'assign', {
enumerable: false,
configurable: true,
writable: true,
value: function(target) {
'use strict';
if (target === undefined || target === null) {
throw new TypeError('Cannot convert first argument to object');
}
var to = Object(target);
for (var i = 1; i < arguments.length; i++) {
var nextSource = arguments[i];
if (nextSource === undefined || nextSource === null) {
continue;
}
nextSource = Object(nextSource);
var keysArray = Object.keys(nextSource);
for (var nextIndex = 0, len = keysArray.length; nextIndex < len; nextIndex++) {
var nextKey = keysArray[nextIndex];
var desc = Object.getOwnPropertyDescriptor(nextSource, nextKey);
if (desc !== undefined && desc.enumerable) {
to[nextKey] = nextSource[nextKey];
}
}
}
return to;
}
});
}

There are many answers, but none that mentions Object.create from ECMAScript 5, which admittedly does not give you an exact copy, but sets the source as the prototype of the new object.
Thus, this is not an exact answer to the question, but it is a one-line solution and thus elegant. And it works best for 2 cases:
Where such inheritance is useful (duh!)
Where the source object won't be modified, thus making the relation between the 2 objects a non issue.
Example:
var foo = { a : 1 };
var bar = Object.create(foo);
foo.a; // 1
bar.a; // 1
foo.a = 2;
bar.a; // 2 - prototype changed
bar.a = 3;
foo.a; // Still 2, since setting bar.a makes it an "own" property
Why do I consider this solution to be superior? It's native, thus no looping, no recursion. However, older browsers will need a polyfill.

There are several issues with most solutions on the internet. So I decided to make a follow-up, which includes, why the accepted answer shouldn't be accepted.
starting situation
I want to deep-copy a Javascript Object with all of its children and their children and so on. But since I'm not kind of a normal developer, my Object has normal properties, circular structures and even nested objects.
So let's create a circular structure and a nested object first.
function Circ() {
this.me = this;
}
function Nested(y) {
this.y = y;
}
Let's bring everything together in an Object named a.
var a = {
x: 'a',
circ: new Circ(),
nested: new Nested('a')
};
Next, we want to copy a into a variable named b and mutate it.
var b = a;
b.x = 'b';
b.nested.y = 'b';
You know what happened here because if not you wouldn't even land on this great question.
console.log(a, b);
a --> Object {
x: "b",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
b --> Object {
x: "b",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
Now let's find a solution.
JSON
The first attempt I tried was using JSON.
var b = JSON.parse( JSON.stringify( a ) );
b.x = 'b';
b.nested.y = 'b';
Don't waste too much time on it, you'll get TypeError: Converting circular structure to JSON.
Recursive copy (the accepted "answer")
Let's have a look at the accepted answer.
function cloneSO(obj) {
// Handle the 3 simple types, and null or undefined
if (null == obj || "object" != typeof obj) return obj;
// Handle Date
if (obj instanceof Date) {
var copy = new Date();
copy.setTime(obj.getTime());
return copy;
}
// Handle Array
if (obj instanceof Array) {
var copy = [];
for (var i = 0, len = obj.length; i < len; i++) {
copy[i] = cloneSO(obj[i]);
}
return copy;
}
// Handle Object
if (obj instanceof Object) {
var copy = {};
for (var attr in obj) {
if (obj.hasOwnProperty(attr)) copy[attr] = cloneSO(obj[attr]);
}
return copy;
}
throw new Error("Unable to copy obj! Its type isn't supported.");
}
Looks good, heh? It's a recursive copy of the object and handles other types as well, like Date, but that wasn't a requirement.
var b = cloneSO(a);
b.x = 'b';
b.nested.y = 'b';
Recursion and circular structures doesn't work well together... RangeError: Maximum call stack size exceeded
native solution
After arguing with my co-worker, my boss asked us what happened, and he found a simple solution after some googling. It's called Object.create.
var b = Object.create(a);
b.x = 'b';
b.nested.y = 'b';
This solution was added to Javascript some time ago and even handles circular structure.
console.log(a, b);
a --> Object {
x: "a",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
b --> Object {
x: "b",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
... and you see, it didn't work with the nested structure inside.
polyfill for the native solution
There's a polyfill for Object.create in the older browser just like the IE 8. It's something like recommended by Mozilla, and of course, it's not perfect and results in the same problem as the native solution.
function F() {};
function clonePF(o) {
F.prototype = o;
return new F();
}
var b = clonePF(a);
b.x = 'b';
b.nested.y = 'b';
I've put F outside the scope so we can have a look at what instanceof tells us.
console.log(a, b);
a --> Object {
x: "a",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
b --> F {
x: "b",
circ: Circ {
me: Circ { ... }
},
nested: Nested {
y: "b"
}
}
console.log(typeof a, typeof b);
a --> object
b --> object
console.log(a instanceof Object, b instanceof Object);
a --> true
b --> true
console.log(a instanceof F, b instanceof F);
a --> false
b --> true
Same problem as the native solution, but a little bit worse output.
the better (but not perfect) solution
When digging around, I found a similar question (In Javascript, when performing a deep copy, how do I avoid a cycle, due to a property being "this"?) to this one, but with a way better solution.
function cloneDR(o) {
const gdcc = "__getDeepCircularCopy__";
if (o !== Object(o)) {
return o; // primitive value
}
var set = gdcc in o,
cache = o[gdcc],
result;
if (set && typeof cache == "function") {
return cache();
}
// else
o[gdcc] = function() { return result; }; // overwrite
if (o instanceof Array) {
result = [];
for (var i=0; i<o.length; i++) {
result[i] = cloneDR(o[i]);
}
} else {
result = {};
for (var prop in o)
if (prop != gdcc)
result[prop] = cloneDR(o[prop]);
else if (set)
result[prop] = cloneDR(cache);
}
if (set) {
o[gdcc] = cache; // reset
} else {
delete o[gdcc]; // unset again
}
return result;
}
var b = cloneDR(a);
b.x = 'b';
b.nested.y = 'b';
And let's have a look at the output...
console.log(a, b);
a --> Object {
x: "a",
circ: Object {
me: Object { ... }
},
nested: Object {
y: "a"
}
}
b --> Object {
x: "b",
circ: Object {
me: Object { ... }
},
nested: Object {
y: "b"
}
}
console.log(typeof a, typeof b);
a --> object
b --> object
console.log(a instanceof Object, b instanceof Object);
a --> true
b --> true
console.log(a instanceof F, b instanceof F);
a --> false
b --> false
The requirements are matched, but there are still some smaller issues, including changing the instance of nested and circ to Object.
The structure of trees that share a leaf won't be copied, they will become two independent leaves:
[Object] [Object]
/ \ / \
/ \ / \
|/_ _\| |/_ _\|
[Object] [Object] ===> [Object] [Object]
\ / | |
\ / | |
_\| |/_ \|/ \|/
[Object] [Object] [Object]
conclusion
The last solution using recursion and a cache, may not be the best, but it's a real deep-copy of the object. It handles simple properties, circular structures and nested object, but it will mess up the instance of them while cloning.
jsfiddle

If you're okay with a shallow copy, the underscore.js library has a clone method.
y = _.clone(x);
or you can extend it like
copiedObject = _.extend({},originalObject);

OK, imagine you have this object below and you want to clone it:
let obj = {a:1, b:2, c:3}; //ES6
or
var obj = {a:1, b:2, c:3}; //ES5
the answer is mainly depeneds on which ECMAscript you using, in ES6+, you can simply use Object.assign to do the clone:
let cloned = Object.assign({}, obj); //new {a:1, b:2, c:3};
or using spread operator like this:
let cloned = {...obj}; //new {a:1, b:2, c:3};
But if you using ES5, you can use few methods, but the JSON.stringify, just make sure you not using for a big chunk of data to copy, but it could be one line handy way in many cases, something like this:
let cloned = JSON.parse(JSON.stringify(obj));
//new {a:1, b:2, c:3};, can be handy, but avoid using on big chunk of data over and over

Update 06 July 2020
There are three (3) ways to clone objects in JavaScript. As objects in JavaScript are reference values, you can't simply just copy using the =.
The ways are:
const food = { food: 'apple', drink: 'milk' }
// 1. Using the "Spread"
// ------------------
{ ...food }
// 2. Using "Object.assign"
// ------------------
Object.assign({}, food)
// 3. "JSON"
// ------------------
JSON.parse(JSON.stringify(food))
// RESULT:
// { food: 'apple', drink: 'milk' }
This can be used as a reference summary.

One particularly inelegant solution is to use JSON encoding to make deep copies of objects that do not have member methods. The methodology is to JSON encode your target object, then by decoding it, you get the copy you are looking for. You can decode as many times as you want to make as many copies as you need.
Of course, functions do not belong in JSON, so this only works for objects without member methods.
This methodology was perfect for my use case, since I'm storing JSON blobs in a key-value store, and when they are exposed as objects in a JavaScript API, each object actually contains a copy of the original state of the object so we can calculate the delta after the caller has mutated the exposed object.
var object1 = {key:"value"};
var object2 = object1;
object2 = JSON.stringify(object1);
object2 = JSON.parse(object2);
object2.key = "a change";
console.log(object1);// returns value

You can simply use a spread property to copy an object without references. But be careful (see comments), the 'copy' is just on the lowest object/array level. Nested properties are still references!
Complete clone:
let x = {a: 'value1'}
let x2 = {...x}
// => mutate without references:
x2.a = 'value2'
console.log(x.a) // => 'value1'
Clone with references on second level:
const y = {a: {b: 'value3'}}
const y2 = {...y}
// => nested object is still a references:
y2.a.b = 'value4'
console.log(y.a.b) // => 'value4'
JavaScript actually does not support deep clones natively. Use an utility function. For example Ramda:
http://ramdajs.com/docs/#clone

const objClone = { ...obj };
Be aware that nested objects are still copied as a reference.

From this article: How to copy arrays and objects in Javascript by Brian Huisman:
Object.prototype.clone = function() {
var newObj = (this instanceof Array) ? [] : {};
for (var i in this) {
if (i == 'clone') continue;
if (this[i] && typeof this[i] == "object") {
newObj[i] = this[i].clone();
} else newObj[i] = this[i]
} return newObj;
};

For those using AngularJS, there is also direct method for cloning or extending of the objects in this library.
var destination = angular.copy(source);
or
angular.copy(source, destination);
More in angular.copy documentation...

function clone(obj) {
if(obj == null || typeof(obj) != 'object')
return obj;
var temp = new obj.constructor();
for(var key in obj)
temp[key] = clone(obj[key]);
return temp;
}

A.Levy's answer is almost complete, here is my little contribution: there is a way how to handle recursive references, see this line
if(this[attr]==this) copy[attr] = copy;
If the object is XML DOM element, we must use cloneNode instead
if(this.cloneNode) return this.cloneNode(true);
Inspired by A.Levy's exhaustive study and Calvin's prototyping approach, I offer this solution:
Object.prototype.clone = function() {
if(this.cloneNode) return this.cloneNode(true);
var copy = this instanceof Array ? [] : {};
for(var attr in this) {
if(typeof this[attr] == "function" || this[attr]==null || !this[attr].clone)
copy[attr] = this[attr];
else if(this[attr]==this) copy[attr] = copy;
else copy[attr] = this[attr].clone();
}
return copy;
}
Date.prototype.clone = function() {
var copy = new Date();
copy.setTime(this.getTime());
return copy;
}
Number.prototype.clone =
Boolean.prototype.clone =
String.prototype.clone = function() {
return this;
}
See also Andy Burke's note in the answers.

Performance
Today 2020.04.30 I perform tests of chosen solutions on Chrome v81.0, Safari v13.1 and Firefox v75.0 on MacOs High Sierra v10.13.6.
I focus on speed of copy DATA (object with simple type fields, not methods etc.). The solutions A-I can make only shallow copy, solutions J-U can make deep copy.
Results for shallow copy
solution {...obj} (A) is fastest on chrome and firefox and medium fast on safari
solution based on Object.assign (B) is fast on all browsers
jQuery (E) and lodash (F,G,H) solutions are medium/quite fast
solution JSON.parse/stringify (K) is quite slow
solutions D and U are slow on all browsers
Results for deep copy
solution Q is fastest on all browsers
jQuery (L) and lodash (J) are medium fast
solution JSON.parse/stringify (K) is quite slow
solution U is slowest on all browsers
lodash (J) and solution U crash on Chrome for 1000 level deep object
Details
For choosen solutions:
A
B
C(my)
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U,
I perform 4 tests
shallow-small: object with 10 non-nested fields - you can run it HERE
shallow-big: object with 1000 non-nested fields - you can run it HERE
deep-small: object with 10 levels-nested fields - you can run it HERE
deep-big: object with 1000 levels-nested fields - you can run it HERE
Objects used in tests are show in below snippet
let obj_ShallowSmall = {
field0: false,
field1: true,
field2: 1,
field3: 0,
field4: null,
field5: [],
field6: {},
field7: "text7",
field8: "text8",
}
let obj_DeepSmall = {
level0: {
level1: {
level2: {
level3: {
level4: {
level5: {
level6: {
level7: {
level8: {
level9: [[[[[[[[[['abc']]]]]]]]]],
}}}}}}}}},
};
let obj_ShallowBig = Array(1000).fill(0).reduce((a,c,i) => (a['field'+i]=getField(i),a) ,{});
let obj_DeepBig = genDeepObject(1000);
// ------------------
// Show objects
// ------------------
console.log('obj_ShallowSmall:',JSON.stringify(obj_ShallowSmall));
console.log('obj_DeepSmall:',JSON.stringify(obj_DeepSmall));
console.log('obj_ShallowBig:',JSON.stringify(obj_ShallowBig));
console.log('obj_DeepBig:',JSON.stringify(obj_DeepBig));
// ------------------
// HELPERS
// ------------------
function getField(k) {
let i=k%10;
if(i==0) return false;
if(i==1) return true;
if(i==2) return k;
if(i==3) return 0;
if(i==4) return null;
if(i==5) return [];
if(i==6) return {};
if(i>=7) return "text"+k;
}
function genDeepObject(N) {
// generate: {level0:{level1:{...levelN: {end:[[[...N-times...['abc']...]]] }}}...}}}
let obj={};
let o=obj;
let arr = [];
let a=arr;
for(let i=0; i<N; i++) {
o['level'+i]={};
o=o['level'+i];
let aa=[];
a.push(aa);
a=aa;
}
a[0]='abc';
o['end']=arr;
return obj;
}
Below snippet presents tested solutions and shows differences between them
function A(obj) {
return {...obj}
}
function B(obj) {
return Object.assign({}, obj);
}
function C(obj) {
return Object.keys(obj).reduce( (a,c) => (a[c]=obj[c], a), {})
}
function D(obj) {
let copyOfObject = {};
Object.defineProperties(copyOfObject, Object.getOwnPropertyDescriptors(obj));
return copyOfObject;
}
function E(obj) {
return jQuery.extend({}, obj) // shallow
}
function F(obj) {
return _.clone(obj);
}
function G(obj) {
return _.clone(obj,true);
}
function H(obj) {
return _.extend({},obj);
}
function I(obj) {
if (null == obj || "object" != typeof obj) return obj;
var copy = obj.constructor();
for (var attr in obj) {
if (obj.hasOwnProperty(attr)) copy[attr] = obj[attr];
}
return copy;
}
function J(obj) {
return _.cloneDeep(obj,true);
}
function K(obj) {
return JSON.parse(JSON.stringify(obj));
}
function L(obj) {
return jQuery.extend(true, {}, obj) // deep
}
function M(obj) {
if(obj == null || typeof(obj) != 'object')
return obj;
var temp = new obj.constructor();
for(var key in obj)
temp[key] = M(obj[key]);
return temp;
}
function N(obj) {
let EClone = function(obj) {
var newObj = (obj instanceof Array) ? [] : {};
for (var i in obj) {
if (i == 'EClone') continue;
if (obj[i] && typeof obj[i] == "object") {
newObj[i] = EClone(obj[i]);
} else newObj[i] = obj[i]
} return newObj;
};
return EClone(obj);
};
function O(obj) {
if (obj == null || typeof obj != "object") return obj;
if (obj.constructor != Object && obj.constructor != Array) return obj;
if (obj.constructor == Date || obj.constructor == RegExp || obj.constructor == Function ||
obj.constructor == String || obj.constructor == Number || obj.constructor == Boolean)
return new obj.constructor(obj);
let to = new obj.constructor();
for (var name in obj)
{
to[name] = typeof to[name] == "undefined" ? O(obj[name], null) : to[name];
}
return to;
}
function P(obj) {
function clone(target, source){
for(let key in source){
// Use getOwnPropertyDescriptor instead of source[key] to prevent from trigering setter/getter.
let descriptor = Object.getOwnPropertyDescriptor(source, key);
if(descriptor.value instanceof String){
target[key] = new String(descriptor.value);
}
else if(descriptor.value instanceof Array){
target[key] = clone([], descriptor.value);
}
else if(descriptor.value instanceof Object){
let prototype = Reflect.getPrototypeOf(descriptor.value);
let cloneObject = clone({}, descriptor.value);
Reflect.setPrototypeOf(cloneObject, prototype);
target[key] = cloneObject;
}
else {
Object.defineProperty(target, key, descriptor);
}
}
let prototype = Reflect.getPrototypeOf(source);
Reflect.setPrototypeOf(target, prototype);
return target;
}
return clone({},obj);
}
function Q(obj) {
var copy;
// Handle the 3 simple types, and null or undefined
if (null == obj || "object" != typeof obj) return obj;
// Handle Date
if (obj instanceof Date) {
copy = new Date();
copy.setTime(obj.getTime());
return copy;
}
// Handle Array
if (obj instanceof Array) {
copy = [];
for (var i = 0, len = obj.length; i < len; i++) {
copy[i] = Q(obj[i]);
}
return copy;
}
// Handle Object
if (obj instanceof Object) {
copy = {};
for (var attr in obj) {
if (obj.hasOwnProperty(attr)) copy[attr] = Q(obj[attr]);
}
return copy;
}
throw new Error("Unable to copy obj! Its type isn't supported.");
}
function R(obj) {
const gdcc = "__getDeepCircularCopy__";
if (obj !== Object(obj)) {
return obj; // primitive value
}
var set = gdcc in obj,
cache = obj[gdcc],
result;
if (set && typeof cache == "function") {
return cache();
}
// else
obj[gdcc] = function() { return result; }; // overwrite
if (obj instanceof Array) {
result = [];
for (var i=0; i<obj.length; i++) {
result[i] = R(obj[i]);
}
} else {
result = {};
for (var prop in obj)
if (prop != gdcc)
result[prop] = R(obj[prop]);
else if (set)
result[prop] = R(cache);
}
if (set) {
obj[gdcc] = cache; // reset
} else {
delete obj[gdcc]; // unset again
}
return result;
}
function S(obj) {
const cache = new WeakMap(); // Map of old - new references
function copy(object) {
if (typeof object !== 'object' ||
object === null ||
object instanceof HTMLElement
)
return object; // primitive value or HTMLElement
if (object instanceof Date)
return new Date().setTime(object.getTime());
if (object instanceof RegExp)
return new RegExp(object.source, object.flags);
if (cache.has(object))
return cache.get(object);
const result = object instanceof Array ? [] : {};
cache.set(object, result); // store reference to object before the recursive starts
if (object instanceof Array) {
for(const o of object) {
result.push(copy(o));
}
return result;
}
const keys = Object.keys(object);
for (const key of keys)
result[key] = copy(object[key]);
return result;
}
return copy(obj);
}
function T(obj){
var clonedObjectsArray = [];
var originalObjectsArray = []; //used to remove the unique ids when finished
var next_objid = 0;
function objectId(obj) {
if (obj == null) return null;
if (obj.__obj_id == undefined){
obj.__obj_id = next_objid++;
originalObjectsArray[obj.__obj_id] = obj;
}
return obj.__obj_id;
}
function cloneRecursive(obj) {
if (null == obj || typeof obj == "string" || typeof obj == "number" || typeof obj == "boolean") return obj;
// Handle Date
if (obj instanceof Date) {
var copy = new Date();
copy.setTime(obj.getTime());
return copy;
}
// Handle Array
if (obj instanceof Array) {
var copy = [];
for (var i = 0; i < obj.length; ++i) {
copy[i] = cloneRecursive(obj[i]);
}
return copy;
}
// Handle Object
if (obj instanceof Object) {
if (clonedObjectsArray[objectId(obj)] != undefined)
return clonedObjectsArray[objectId(obj)];
var copy;
if (obj instanceof Function)//Handle Function
copy = function(){return obj.apply(this, arguments);};
else
copy = {};
clonedObjectsArray[objectId(obj)] = copy;
for (var attr in obj)
if (attr != "__obj_id" && obj.hasOwnProperty(attr))
copy[attr] = cloneRecursive(obj[attr]);
return copy;
}
throw new Error("Unable to copy obj! Its type isn't supported.");
}
var cloneObj = cloneRecursive(obj);
//remove the unique ids
for (var i = 0; i < originalObjectsArray.length; i++)
{
delete originalObjectsArray[i].__obj_id;
};
return cloneObj;
}
function U(obj) {
/*
Deep copy objects by value rather than by reference,
exception: `Proxy`
*/
const seen = new WeakMap()
return clone(obj)
function defineProp(object, key, descriptor = {}, copyFrom = {}) {
const { configurable: _configurable, writable: _writable }
= Object.getOwnPropertyDescriptor(object, key)
|| { configurable: true, writable: true }
const test = _configurable // Can redefine property
&& (_writable === undefined || _writable) // Can assign to property
if (!test || arguments.length <= 2) return test
const basisDesc = Object.getOwnPropertyDescriptor(copyFrom, key)
|| { configurable: true, writable: true } // Custom…
|| {}; // …or left to native default settings
["get", "set", "value", "writable", "enumerable", "configurable"]
.forEach(attr =>
descriptor[attr] === undefined &&
(descriptor[attr] = basisDesc[attr])
)
const { get, set, value, writable, enumerable, configurable }
= descriptor
return Object.defineProperty(object, key, {
enumerable, configurable, ...get || set
? { get, set } // Accessor descriptor
: { value, writable } // Data descriptor
})
}
function clone(object) {
if (object !== Object(object)) return object /*
—— Check if the object belongs to a primitive data type */
if (object instanceof Node) return object.cloneNode(true) /*
—— Clone DOM trees */
let _object // The clone of object
switch (object.constructor) {
case Array:
case Object:
_object = cloneObject(object)
break
case Date:
_object = new Date(+object)
break
case Function:
const fnStr = String(object)
_object = new Function("return " +
(/^(?!function |[^{]+?=>)[^(]+?\(/.test(fnStr)
? "function " : ""
) + fnStr
)()
copyPropDescs(_object, object)
break
case RegExp:
_object = new RegExp(object)
break
default:
switch (Object.prototype.toString.call(object.constructor)) {
// // Stem from:
case "[object Function]": // `class`
case "[object Undefined]": // `Object.create(null)`
_object = cloneObject(object)
break
default: // `Proxy`
_object = object
}
}
return _object
}
function cloneObject(object) {
if (seen.has(object)) return seen.get(object) /*
—— Handle recursive references (circular structures) */
const _object = Array.isArray(object)
? []
: Object.create(Object.getPrototypeOf(object)) /*
—— Assign [[Prototype]] for inheritance */
seen.set(object, _object) /*
—— Make `_object` the associative mirror of `object` */
Reflect.ownKeys(object).forEach(key =>
defineProp(_object, key, { value: clone(object[key]) }, object)
)
return _object
}
function copyPropDescs(target, source) {
Object.defineProperties(target,
Object.getOwnPropertyDescriptors(source)
)
}
}
// ------------------------
// Test properties
// ------------------------
console.log(` shallow deep func circ undefined date RegExp bigInt`)
log(A);
log(B);
log(C);
log(D);
log(E);
log(F);
log(G);
log(H);
log(I);
log(J);
log(K);
log(L);
log(M);
log(N);
log(O);
log(P);
log(Q);
log(R);
log(S);
log(T);
log(U);
console.log(` shallow deep func circ undefined date RegExp bigInt
----
LEGEND:
shallow - solution create shallow copy
deep - solution create deep copy
func - solution copy functions
circ - solution can copy object with circular references
undefined - solution copy fields with undefined value
date - solution can copy date
RegExp - solution can copy fields with regular expressions
bigInt - solution can copy BigInt
`)
// ------------------------
// Helper functions
// ------------------------
function deepCompare(obj1,obj2) {
return JSON.stringify(obj1)===JSON.stringify(obj2);
}
function getCase() { // pure data case
return {
undef: undefined,
bool: true, num: 1, str: "txt1",
e1: null, e2: [], e3: {}, e4: 0, e5: false,
arr: [ false, 2, "txt3", null, [], {},
[ true,4,"txt5",null, [], {}, [true,6,"txt7",null,[],{} ],
{bool: true,num: 8, str: "txt9", e1:null, e2:[] ,e3:{} ,e4: 0, e5: false}
],
{bool: true,num: 10, str: "txt11", e1:null, e2:[] ,e3:{} ,e4: 0, e5: false}
],
obj: {
bool: true, num: 12, str: "txt13",
e1: null, e2: [], e3: {}, e4: 0, e5: false,
arr: [true,14,"txt15",null,[],{} ],
obj: {
bool: true, num: 16, str: "txt17",
e1: null, e2: [], e3: {}, e4: 0, e5: false,
arr: [true,18,"txt19",null,[],{} ],
obj: {bool: true,num: 20, str: "txt21", e1:null, e2:[] ,e3:{} ,e4: 0, e5: false}
}
}
};
}
function check(org, copy, field, newValue) {
copy[field] = newValue;
return deepCompare(org,copy);
}
function testFunc(f) {
let o = { a:1, fun: (i,j)=> i+j };
let c = f(o);
let val = false
try{
val = c.fun(3,4)==7;
} catch(e) { }
return val;
}
function testCirc(f) {
function Circ() {
this.me = this;
}
var o = {
x: 'a',
circ: new Circ(),
obj_circ: null,
};
o.obj_circ = o;
let val = false;
try{
let c = f(o);
val = (o.obj_circ == o) && (o.circ == o.circ.me);
} catch(e) { }
return val;
}
function testRegExp(f) {
let o = {
re: /a[0-9]+/,
};
let val = false;
try{
let c = f(o);
val = (String(c.re) == String(/a[0-9]+/));
} catch(e) { }
return val;
}
function testDate(f) {
let o = {
date: new Date(),
};
let val = false;
try{
let c = f(o);
val = (+new Date(c.date) == +new Date(o.date));
} catch(e) { }
return val;
}
function testBigInt(f) {
let val = false;
try{
let o = {
big: 123n,
};
let c = f(o);
val = o.big == c.big;
} catch(e) { }
return val;
}
function log(f) {
let o = getCase(); // orginal object
let oB = getCase(); // "backup" used for shallow valid test
let c1 = f(o); // copy 1 for reference
let c2 = f(o); // copy 2 for test shallow values
let c3 = f(o); // copy 3 for test deep values
let is_proper_copy = deepCompare(c1,o); // shoud be true
// shallow changes
let testShallow =
[ ['bool',false],['num',666],['str','xyz'],['arr',[]],['obj',{}] ]
.reduce((acc,curr)=> acc && check(c1,c2,curr[0], curr[1]), true );
// should be true (original object shoud not have changed shallow fields)
let is_valid = deepCompare(o,oB);
// deep test (intruduce some change)
if (c3.arr[6]) c3.arr[6][7].num = 777;
let diff_shallow = !testShallow; // shoud be true (shallow field was copied)
let diff_deep = !deepCompare(c1,c3); // shoud be true (deep field was copied)
let can_copy_functions = testFunc(f);
let can_copy_circular = testCirc(f);
let can_copy_regexp = testRegExp(f);
let can_copy_date = testDate(f);
let can_copy_bigInt = testBigInt(f);
let has_undefined = 'undef' in c1; // field with undefined value is copied?
let is_ok = is_valid && is_proper_copy;
let b=(bool) => (bool+'').padEnd(5,' '); // bool value to formated string
testFunc(f);
if(is_ok) {
console.log(`${f.name} ${b(diff_shallow)} ${b(diff_deep)} ${b(can_copy_functions)} ${b(can_copy_circular)} ${b(has_undefined)} ${b(can_copy_date)} ${b(can_copy_regexp)} ${b(can_copy_bigInt)}`)
} else {
console.log(`${f.name}: INVALID ${is_valid} ${is_proper_copy}`,{c1})
}
}
<script src="https://code.jquery.com/jquery-3.5.0.min.js" integrity="sha256-xNzN2a4ltkB44Mc/Jz3pT4iU1cmeR0FkXs4pru/JxaQ=" crossorigin="anonymous"></script>
<script src="https://cdn.jsdelivr.net/npm/lodash#4.17.15/lodash.min.js"></script>
This snippet only presents tested solutions and show differences between them (but it no make performence tests)
Below there are example results for Chrome for shallow-big object

Using Lodash:
var y = _.clone(x, true);

In ES-6 you can simply use Object.assign(...).
Ex:
let obj = {person: 'Thor Odinson'};
let clone = Object.assign({}, obj);
A good reference is here:
https://googlechrome.github.io/samples/object-assign-es6/

Interested in cloning simple objects:
JSON.parse(JSON.stringify(json_original));
Source : How to copy JavaScript object to new variable NOT by reference?

let clone = Object.assign( Object.create( Object.getPrototypeOf(obj)), obj)
ES6 solution if you want to (shallow) clone a class instance and not just a property object.

Structured Cloning
2022 update: The structuredClone() global function is already available in Node 17, Deno 1.14, and most major browsers (see Can I Use).
You can use the same structured clone mechanism that the HTML standard includes for sending data between realms.
const clone = structuredClone(original);
See the other answer for more details.

You can clone an object and remove any reference from the previous one using a single line of code. Simply do:
var obj1 = { text: 'moo1' };
var obj2 = Object.create(obj1); // Creates a new clone without references
obj2.text = 'moo2'; // Only updates obj2's text property
console.log(obj1, obj2); // Outputs: obj1: {text:'moo1'}, obj2: {text:'moo2'}
For browsers / engines that do not currently support Object.create you can use this polyfill:
// Polyfill Object.create if it does not exist
if (!Object.create) {
Object.create = function (o) {
var F = function () {};
F.prototype = o;
return new F();
};
}

New answer to an old question! If you have the pleasure of having using ECMAScript 2016 (ES6) with Spread Syntax, it's easy.
keepMeTheSame = {first: "Me!", second: "You!"};
cloned = {...keepMeTheSame}
This provides a clean method for a shallow copy of an object. Making a deep copy, meaning makign a new copy of every value in every recursively nested object, requires on of the heavier solutions above.
JavaScript keeps evolving.

I think there is a simple and working answer. In deep copying there are two concerns:
Keep properties independent to each other.
And keep the methods alive on cloned object.
So I think one simple solution will be to first serialize and deserialize and then do an assign on it to copy functions too.
let deepCloned = JSON.parse(JSON.stringify(source));
let merged = Object.assign({}, source);
Object.assign(merged, deepCloned);
Although this question has many answers, I hope this one helps too.

For a deep copy and clone, JSON.stringify then JSON.parse the object:
obj = { a: 0 , b: { c: 0}};
let deepClone = JSON.parse(JSON.stringify(obj));
obj.a = 5;
obj.b.c = 5;
console.log(JSON.stringify(deepClone)); // { a: 0, b: { c: 0}}

(The following was mainly an integration of #Maciej Bukowski, #A. Levy, #Jan Turoň, #Redu's answers, and #LeviRoberts, #RobG's comments, many thanks to them!!!)
Deep copy? — YES! (mostly);
Shallow copy? — NO! (except Proxy).
I sincerely welcome everyone to test clone().
In addition, defineProp() is designed to easily and quickly (re)define or copy any type of descriptor.
Function
function clone(object) {
/*
Deep copy objects by value rather than by reference,
exception: `Proxy`
*/
const seen = new WeakMap()
return clone(object)
function clone(object) {
if (object !== Object(object)) return object /*
—— Check if the object belongs to a primitive data type */
if (object instanceof Node) return object.cloneNode(true) /*
—— Clone DOM trees */
let _object // The clone of object
switch (object.constructor) {
case Array:
case Object:
_object = cloneObject(object)
break
case Date:
_object = new Date(+object)
break
case Function:
_object = copyFn(object)
break
case RegExp:
_object = new RegExp(object)
break
default:
switch (Object.prototype.toString.call(object.constructor)) {
// // Stem from:
case "[object Function]":
switch (object[Symbol.toStringTag]) {
case undefined:
_object = cloneObject(object) // `class`
break
case "AsyncFunction":
case "GeneratorFunction":
case "AsyncGeneratorFunction":
_object = copyFn(object)
break
default:
_object = object
}
break
case "[object Undefined]": // `Object.create(null)`
_object = cloneObject(object)
break
default:
_object = object // `Proxy`
}
}
return _object
}
function cloneObject(object) {
if (seen.has(object)) return seen.get(object) /*
—— Handle recursive references (circular structures) */
const _object = Array.isArray(object)
? []
: Object.create(Object.getPrototypeOf(object)) /*
—— Assign [[Prototype]] for inheritance */
seen.set(object, _object) /*
—— Make `_object` the associative mirror of `object` */
Reflect.ownKeys(object).forEach(key =>
defineProp(_object, key, { value: clone(object[key]) }, object)
)
return _object
}
}
function copyPropDescs(target, source) {
Object.defineProperties(target,
Object.getOwnPropertyDescriptors(source)
)
}
function convertFnToStr(fn) {
let fnStr = String(fn)
if (fn.name.startsWith("[")) // isSymbolKey
fnStr = fnStr.replace(/\[Symbol\..+?\]/, '')
fnStr = /^(?!(async )?(function\b|[^{]+?=>))[^(]+?\(/.test(fnStr)
? fnStr.replace(/^(async )?(\*)?/, "$1function$2 ") : fnStr
return fnStr
}
function copyFn(fn) {
const newFn = new Function(`return ${convertFnToStr(fn)}`)()
copyPropDescs(newFn, fn)
return newFn
}
function defineProp(object, key, descriptor = {}, copyFrom = {}) {
const { configurable: _configurable, writable: _writable }
= Object.getOwnPropertyDescriptor(object, key)
|| { configurable: true, writable: true }
const test = _configurable // Can redefine property
&& (_writable === undefined || _writable) // Can assign to property
if (!test || arguments.length <= 2) return test
const basisDesc = Object.getOwnPropertyDescriptor(copyFrom, key)
|| { configurable: true, writable: true } // Custom…
|| {}; // …or left to native default settings
["get", "set", "value", "writable", "enumerable", "configurable"]
.forEach(attr =>
descriptor[attr] === undefined &&
(descriptor[attr] = basisDesc[attr])
)
const { get, set, value, writable, enumerable, configurable }
= descriptor
return Object.defineProperty(object, key, {
enumerable, configurable, ...get || set
? { get, set } // Accessor descriptor
: { value, writable } // Data descriptor
})
}
// Tests
const obj0 = {
u: undefined,
nul: null,
t: true,
num: 9,
str: "",
sym: Symbol("symbol"),
[Symbol("e")]: Math.E,
arr: [[0], [1, 2]],
d: new Date(),
re: /f/g,
get g() { return 0 },
o: {
n: 0,
o: { f: function (...args) { } }
},
f: {
getAccessorStr(object) {
return []
.concat(...
Object.values(Object.getOwnPropertyDescriptors(object))
.filter(desc => desc.writable === undefined)
.map(desc => Object.values(desc))
)
.filter(prop => typeof prop === "function")
.map(String)
},
f0: function f0() { },
f1: function () { },
f2: a => a / (a + 1),
f3: () => 0,
f4(params) { return param => param + params },
f5: (a, b) => ({ c = 0 } = {}) => a + b + c
}
}
defineProp(obj0, "s", { set(v) { this._s = v } })
defineProp(obj0.arr, "tint", { value: { is: "non-enumerable" } })
obj0.arr[0].name = "nested array"
let obj1 = clone(obj0)
obj1.o.n = 1
obj1.o.o.g = function g(a = 0, b = 0) { return a + b }
obj1.arr[1][1] = 3
obj1.d.setTime(+obj0.d + 60 * 1000)
obj1.arr.tint.is = "enumerable? no"
obj1.arr[0].name = "a nested arr"
defineProp(obj1, "s", { set(v) { this._s = v + 1 } })
defineProp(obj1.re, "multiline", { value: true })
console.log("\n\n" + "-".repeat(2 ** 6))
console.log(">:>: Test - Routinely")
console.log("obj0:\n ", JSON.stringify(obj0))
console.log("obj1:\n ", JSON.stringify(obj1))
console.log()
console.log("obj0:\n ", obj0)
console.log("obj1:\n ", obj1)
console.log()
console.log("obj0\n ",
".arr.tint:", obj0.arr.tint, "\n ",
".arr[0].name:", obj0.arr[0].name
)
console.log("obj1\n ",
".arr.tint:", obj1.arr.tint, "\n ",
".arr[0].name:", obj1.arr[0].name
)
console.log()
console.log("Accessor-type descriptor\n ",
"of obj0:", obj0.f.getAccessorStr(obj0), "\n ",
"of obj1:", obj1.f.getAccessorStr(obj1), "\n ",
"set (obj0 & obj1) .s :", obj0.s = obj1.s = 0, "\n ",
" → (obj0 , obj1) ._s:", obj0._s, ",", obj1._s
)
console.log("—— obj0 has not been interfered.")
console.log("\n\n" + "-".repeat(2 ** 6))
console.log(">:>: Test - More kinds of functions")
const fnsForTest = {
f(_) { return _ },
func: _ => _,
aFunc: async _ => _,
async function() { },
async asyncFunc() { },
aFn: async function () { },
*gen() { },
async *asyncGen() { },
aG1: async function* () { },
aG2: async function* gen() { },
*[Symbol.iterator]() { yield* Object.keys(this) }
}
console.log(Reflect.ownKeys(fnsForTest).map(k =>
`${String(k)}:
${fnsForTest[k].name}-->
${String(fnsForTest[k])}`
).join("\n"))
const normedFnsStr = `{
f: function f(_) { return _ },
func: _ => _,
aFunc: async _ => _,
function: async function() { },
asyncFunc: async function asyncFunc() { },
aFn: async function () { },
gen: function* gen() { },
asyncGen: async function* asyncGen() { },
aG1: async function* () { },
aG2: async function* gen() { },
[Symbol.iterator]: function* () { yield* Object.keys(this) }
}`
const copiedFnsForTest = clone(fnsForTest)
console.log("fnsForTest:", fnsForTest)
console.log("fnsForTest (copied):", copiedFnsForTest)
console.log("fnsForTest (normed str):", eval(`(${normedFnsStr})`))
console.log("Comparison of fnsForTest and its clone:",
Reflect.ownKeys(fnsForTest).map(k =>
[k, fnsForTest[k] === copiedFnsForTest[k]]
)
)
console.log("\n\n" + "-".repeat(2 ** 6))
console.log(">:>: Test - Circular structures")
obj0.o.r = {}
obj0.o.r.recursion = obj0.o
obj0.arr[1] = obj0.arr
obj1 = clone(obj0)
console.log("obj0:\n ", obj0)
console.log("obj1:\n ", obj1)
console.log("Clear obj0's recursion:",
obj0.o.r.recursion = null, obj0.arr[1] = 1
)
console.log(
"obj0\n ",
".o.r:", obj0.o.r, "\n ",
".arr:", obj0.arr
)
console.log(
"obj1\n ",
".o.r:", obj1.o.r, "\n ",
".arr:", obj1.arr
)
console.log("—— obj1 has not been interfered.")
console.log("\n\n" + "-".repeat(2 ** 6))
console.log(">:>: Test - Classes")
class Person {
constructor(name) {
this.name = name
}
}
class Boy extends Person { }
Boy.prototype.sex = "M"
const boy0 = new Boy
boy0.hobby = { sport: "spaceflight" }
const boy1 = clone(boy0)
boy1.hobby.sport = "superluminal flight"
boy0.name = "one"
boy1.name = "neo"
console.log("boy0:\n ", boy0)
console.log("boy1:\n ", boy1)
console.log("boy1's prototype === boy0's:",
Object.getPrototypeOf(boy1) === Object.getPrototypeOf(boy0)
)
References
Object.create() | MDN
Object.defineProperties() | MDN
Enumerability and ownership of properties | MDN
TypeError: cyclic object value | MDN
Language tricks used
Conditionally add prop to object

Use lodash _.cloneDeep().
Shallow Copy: lodash _.clone()
A shallow copy can be made by simply copying the reference.
let obj1 = {
a: 0,
b: {
c: 0,
e: {
f: 0
}
}
};
let obj3 = _.clone(obj1);
obj1.a = 4;
obj1.b.c = 4;
obj1.b.e.f = 100;
console.log(JSON.stringify(obj1));
//{"a":4,"b":{"c":4,"e":{"f":100}}}
console.log(JSON.stringify(obj3));
//{"a":0,"b":{"c":4,"e":{"f":100}}}
Deep Copy: lodash _.cloneDeep()
fields are dereferenced: rather than references to objects being copied
let obj1 = {
a: 0,
b: {
c: 0,
e: {
f: 0
}
}
};
let obj3 = _.cloneDeep(obj1);
obj1.a = 100;
obj1.b.c = 100;
obj1.b.e.f = 100;
console.log(JSON.stringify(obj1));
{"a":100,"b":{"c":100,"e":{"f":100}}}
console.log(JSON.stringify(obj3));
{"a":0,"b":{"c":0,"e":{"f":0}}}

Clone Object without reference javascript [duplicate]

This question already has answers here:
What is the most efficient way to deep clone an object in JavaScript?
(67 answers)
Closed 2 years ago.
I have a big object with much data. And i want to clone this in other variable. When i set some param of the instance B has the same result in the original object:
var obj = {a: 25, b: 50, c: 75};
var A = obj;
var B = obj;
A.a = 30;
B.a = 40;
alert(obj.a + " " + A.a + " " + B.a); // 40 40 40
My output should be 25 30 40.
Any ideas?
EDIT
Thanks Everyone. I change the code of dystroy and this is my result:
Object.prototype.clone = Array.prototype.clone = function()
{
if (Object.prototype.toString.call(this) === '[object Array]')
{
var clone = [];
for (var i=0; i<this.length; i++)
clone[i] = this[i].clone();
return clone;
}
else if (typeof(this)=="object")
{
var clone = {};
for (var prop in this)
if (this.hasOwnProperty(prop))
clone[prop] = this[prop].clone();
return clone;
}
else
return this;
}
var obj = {a: 25, b: 50, c: 75};
var A = obj.clone();
var B = obj.clone();
A.a = 30;
B.a = 40;
alert(obj.a + " " + A.a + " " + B.a);
var arr = [25, 50, 75];
var C = arr.clone();
var D = arr.clone();
C[0] = 30;
D[0] = 40;
alert(arr[0] + " " + C[0] + " " + D[0]);

If you use an = statement to assign a value to a var with an object on the right side, javascript will not copy but reference the object.
Spoiler : using JSON.parse(JSON.stringify(obj)) may work but is costly, and might throw a TypeError as in
const a = {};
const b = { a };
a.b = b;
const clone = JSON.parse(JSON.stringify(a));
/* Throws
Uncaught TypeError: Converting circular structure to JSON
--> starting at object with constructor 'Object'
| property 'b' -> object with constructor 'Object'
--- property 'a' closes the circle
at JSON.stringify (<anonymous>)
at <anonymous>:4:6
*/
As of es2015, if you want a shallow copy (clone the object, but keeping deep refences in the inner structure) you can use destructuring :
const obj = { foo: { bar: "baz" } };
const shallowClone = { ...obj };
shallowClone is a new object, but shallowClone.foo holds a reference to the same object as obj.foo.
You can use lodash's clone method, which does the same, if you don't have access to the spread operator.
var obj = {a: 25, b: 50, c: 75};
var A = _.clone(obj);
Or lodash's cloneDeep method if your object has multiple object levels
var obj = {a: 25, b: {a: 1, b: 2}, c: 75};
var A = _.cloneDeep(obj);
Or lodash's merge method if you mean to extend the source object
var obj = {a: 25, b: {a: 1, b: 2}, c: 75};
var A = _.merge({}, obj, {newkey: "newvalue"});
Or you can use jQuerys extend method:
var obj = {a: 25, b: 50, c: 75};
var A = $.extend(true,{},obj);
Here is jQuery 1.11 extend method's source code :
jQuery.extend = jQuery.fn.extend = function() {
var src, copyIsArray, copy, name, options, clone,
target = arguments[0] || {},
i = 1,
length = arguments.length,
deep = false;
// Handle a deep copy situation
if ( typeof target === "boolean" ) {
deep = target;
// skip the boolean and the target
target = arguments[ i ] || {};
i++;
}
// Handle case when target is a string or something (possible in deep copy)
if ( typeof target !== "object" && !jQuery.isFunction(target) ) {
target = {};
}
// extend jQuery itself if only one argument is passed
if ( i === length ) {
target = this;
i--;
}
for ( ; i < length; i++ ) {
// Only deal with non-null/undefined values
if ( (options = arguments[ i ]) != null ) {
// Extend the base object
for ( name in options ) {
src = target[ name ];
copy = options[ name ];
// Prevent never-ending loop
if ( target === copy ) {
continue;
}
// Recurse if we're merging plain objects or arrays
if ( deep && copy && ( jQuery.isPlainObject(copy) || (copyIsArray = jQuery.isArray(copy)) ) ) {
if ( copyIsArray ) {
copyIsArray = false;
clone = src && jQuery.isArray(src) ? src : [];
} else {
clone = src && jQuery.isPlainObject(src) ? src : {};
}
// Never move original objects, clone them
target[ name ] = jQuery.extend( deep, clone, copy );
// Don't bring in undefined values
} else if ( copy !== undefined ) {
target[ name ] = copy;
}
}
}
}
// Return the modified object
return target;
};
var item ={ 'a': 1, 'b': 2}
Object.assign({}, item);

While this isn't cloning, one simple way to get your result is to use the original object as the prototype of a new one.
You can do this using Object.create:
var obj = {a: 25, b: 50, c: 75};
var A = Object.create(obj);
var B = Object.create(obj);
A.a = 30;
B.a = 40;
alert(obj.a + " " + A.a + " " + B.a); // 25 30 40
This creates a new object in A and B that inherits from obj. This means that you can add properties without affecting the original.
To support legacy implementations, you can create a (partial) shim that will work for this simple task.
if (!Object.create)
Object.create = function(proto) {
function F(){}
F.prototype = proto;
return new F;
}
It doesn't emulate all the functionality of Object.create, but it'll fit your needs here.

You could define a clone function.
I use this one :
function goclone(source) {
if (Object.prototype.toString.call(source) === '[object Array]') {
var clone = [];
for (var i=0; i<source.length; i++) {
clone[i] = goclone(source[i]);
}
return clone;
} else if (typeof(source)=="object") {
var clone = {};
for (var prop in source) {
if (source.hasOwnProperty(prop)) {
clone[prop] = goclone(source[prop]);
}
}
return clone;
} else {
return source;
}
}
var B = goclone(A);
It doesn't copy the prototype, functions, and so on. But you should adapt it (and maybe simplify it) for you own need.

A and B reference the same object, so A.a and B.a reference the same property of the same object.
Edit
Here's a "copy" function that may do the job, it can do both shallow and deep clones. Note the caveats. It copies all enumerable properties of an object (not inherited properties), including those with falsey values (I don't understand why other approaches ignore them), it also doesn't copy non–existent properties of sparse arrays.
There is no general copy or clone function because there are many different ideas on what a copy or clone should do in every case. Most rule out host objects, or anything other than Objects or Arrays. This one also copies primitives. What should happen with functions?
So have a look at the following, it's a slightly different approach to others.
/* Only works for native objects, host objects are not
** included. Copies Objects, Arrays, Functions and primitives.
** Any other type of object (Number, String, etc.) will likely give
** unexpected results, e.g. copy(new Number(5)) ==> 0 since the value
** is stored in a non-enumerable property.
**
** Expects that objects have a properly set *constructor* property.
*/
function copy(source, deep) {
var o, prop, type;
if (typeof source != 'object' || source === null) {
// What do to with functions, throw an error?
o = source;
return o;
}
o = new source.constructor();
for (prop in source) {
if (source.hasOwnProperty(prop)) {
type = typeof source[prop];
if (deep && type == 'object' && source[prop] !== null) {
o[prop] = copy(source[prop]);
} else {
o[prop] = source[prop];
}
}
}
return o;
}