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I have been reading about Destructuring assignment introduced in ES6.
What is the purpose of this syntax, why was it introduced, and what are some examples of how it might be used in practice?
What is destructuring assignment ?
The destructuring assignment syntax is a JavaScript expression that makes it possible to unpack values from arrays, or properties from objects, into distinct variables.
- MDN
Advantages
A. Makes code concise and more readable.
B. We can easily avoid repeated destructing expression.
Some use cases
1. To get values in variable from Objects,array
let obj = { 'a': 1,'b': {'b1': '1.1'}}
let {a,b,b:{b1}} = obj
console.log('a--> ' + a, '\nb--> ', b, '\nb1---> ', b1)
let obj2 = { foo: 'foo' };
let { foo: newVarName } = obj2;
console.log(newVarName);
let arr = [1, 2, 3, 4, 5]
let [first, second, ...rest] = arr
console.log(first, '\n', second, '\n', rest)
// Nested extraction is possible too:
let obj3 = { foo: { bar: 'bar' } };
let { foo: { bar } } = obj3;
console.log(bar);
2. To combine an array at any place with another array.
let arr = [2,3,4,5]
let newArr = [0,1,...arr,6,7]
console.log(newArr)
3. To change only desired property in an object
let arr = [{a:1, b:2, c:3},{a:4, b:5, c:6},{a:7, b:8, c:9}]
let op = arr.map( ( {a,...rest}, index) => ({...rest,a:index+10}))
console.log(op)
4. To create a shallow copy of objects
let obj = {a:1,b:2,c:3}
let newObj = {...obj}
newObj.a = 'new Obj a'
console.log('Original Object', obj)
console.log('Shallow copied Object', newObj)
5. To extract values from parameters into standalone variables
// Object destructuring:
const fn = ({ prop }) => {
console.log(prop);
};
fn({ prop: 'foo' });
console.log('------------------');
// Array destructuring:
const fn2 = ([item1, item2]) => {
console.log(item1);
console.log(item2);
};
fn2(['bar', 'baz']);
console.log('------------------');
// Assigning default values to destructured properties:
const fn3 = ({ foo="defaultFooVal", bar }) => {
console.log(foo, bar);
};
fn3({ bar: 'bar' });
6. To get dynamic keys value from object
let obj = {a:1,b:2,c:3}
let key = 'c'
let {[key]:value} = obj
console.log(value)
7. To build an object from other object with some default values
let obj = {a:1,b:2,c:3}
let newObj = (({d=4,...rest} = obj), {d,...rest})
console.log(newObj)
8. To swap values
const b = [1, 2, 3, 4];
[b[0], b[2]] = [b[2], b[0]]; // swap index 0 and 2
console.log(b);
9. To get a subset of an object
9.1 subset of an object:
const obj = {a:1, b:2, c:3},
subset = (({a, c}) => ({a, c}))(obj); // credit to Ivan N for this function
console.log(subset);
9.2 To get a subset of an object using comma operator and destructuring:
const object = { a: 5, b: 6, c: 7 };
const picked = ({a,c}=object, {a,c})
console.log(picked); // { a: 5, c: 7 }
10. To do array to object conversion:
const arr = ["2019", "09", "02"],
date = (([year, day, month]) => ({year, month, day}))(arr);
console.log(date);
11. To set default values in function. (Read this answer for more info )
function someName(element, input,settings={i:"#1d252c", i2:"#fff",...input}){
console.log(settings.i)
console.log(settings.i2)
}
someName('hello', {i:'#123'})
someName('hello', {i2:'#123'})
12. To get properties such as length from an array, function name, number of arguments etc.
let arr = [1,2,3,4,5];
let {length} = arr;
console.log(length);
let func = function dummyFunc(a,b,c) {
return 'A B and C';
}
let {name, length:funcLen} = func;
console.log(name, funcLen);
It is something like what you have can be extracted with the same variable name
The destructuring assignment is a JavaScript expression that makes it possible to unpack values from arrays or properties from objects into distinct variables. Let's get the month values from an array using destructuring assignment
var [one, two, three] = ['orange', 'mango', 'banana'];
console.log(one); // "orange"
console.log(two); // "mango"
console.log(three); // "banana"
and you can get user properties of an object using destructuring assignment,
var {name, age} = {name: 'John', age: 32};
console.log(name); // John
console.log(age); // 32
The De-structured assignment of Javascript is probably an inspiration drawn from Perl language.
This facilitates reuse by avoid writing getter methods or wrapper functions.
One best example that I found very helpful in particular was on reusing functions that return more data than what is required.
If there is a function that returns a list or an array or a json, and we are interested in only the first item of the list or array or json,
then we can simply use the de-structured assignment instead of writing a new wrapper function to extract the interesting data item.
I have been reading about Destructuring assignment introduced in ES6.
What is the purpose of this syntax, why was it introduced, and what are some examples of how it might be used in practice?
What is destructuring assignment ?
The destructuring assignment syntax is a JavaScript expression that makes it possible to unpack values from arrays, or properties from objects, into distinct variables.
- MDN
Advantages
A. Makes code concise and more readable.
B. We can easily avoid repeated destructing expression.
Some use cases
1. To get values in variable from Objects,array
let obj = { 'a': 1,'b': {'b1': '1.1'}}
let {a,b,b:{b1}} = obj
console.log('a--> ' + a, '\nb--> ', b, '\nb1---> ', b1)
let obj2 = { foo: 'foo' };
let { foo: newVarName } = obj2;
console.log(newVarName);
let arr = [1, 2, 3, 4, 5]
let [first, second, ...rest] = arr
console.log(first, '\n', second, '\n', rest)
// Nested extraction is possible too:
let obj3 = { foo: { bar: 'bar' } };
let { foo: { bar } } = obj3;
console.log(bar);
2. To combine an array at any place with another array.
let arr = [2,3,4,5]
let newArr = [0,1,...arr,6,7]
console.log(newArr)
3. To change only desired property in an object
let arr = [{a:1, b:2, c:3},{a:4, b:5, c:6},{a:7, b:8, c:9}]
let op = arr.map( ( {a,...rest}, index) => ({...rest,a:index+10}))
console.log(op)
4. To create a shallow copy of objects
let obj = {a:1,b:2,c:3}
let newObj = {...obj}
newObj.a = 'new Obj a'
console.log('Original Object', obj)
console.log('Shallow copied Object', newObj)
5. To extract values from parameters into standalone variables
// Object destructuring:
const fn = ({ prop }) => {
console.log(prop);
};
fn({ prop: 'foo' });
console.log('------------------');
// Array destructuring:
const fn2 = ([item1, item2]) => {
console.log(item1);
console.log(item2);
};
fn2(['bar', 'baz']);
console.log('------------------');
// Assigning default values to destructured properties:
const fn3 = ({ foo="defaultFooVal", bar }) => {
console.log(foo, bar);
};
fn3({ bar: 'bar' });
6. To get dynamic keys value from object
let obj = {a:1,b:2,c:3}
let key = 'c'
let {[key]:value} = obj
console.log(value)
7. To build an object from other object with some default values
let obj = {a:1,b:2,c:3}
let newObj = (({d=4,...rest} = obj), {d,...rest})
console.log(newObj)
8. To swap values
const b = [1, 2, 3, 4];
[b[0], b[2]] = [b[2], b[0]]; // swap index 0 and 2
console.log(b);
9. To get a subset of an object
9.1 subset of an object:
const obj = {a:1, b:2, c:3},
subset = (({a, c}) => ({a, c}))(obj); // credit to Ivan N for this function
console.log(subset);
9.2 To get a subset of an object using comma operator and destructuring:
const object = { a: 5, b: 6, c: 7 };
const picked = ({a,c}=object, {a,c})
console.log(picked); // { a: 5, c: 7 }
10. To do array to object conversion:
const arr = ["2019", "09", "02"],
date = (([year, day, month]) => ({year, month, day}))(arr);
console.log(date);
11. To set default values in function. (Read this answer for more info )
function someName(element, input,settings={i:"#1d252c", i2:"#fff",...input}){
console.log(settings.i)
console.log(settings.i2)
}
someName('hello', {i:'#123'})
someName('hello', {i2:'#123'})
12. To get properties such as length from an array, function name, number of arguments etc.
let arr = [1,2,3,4,5];
let {length} = arr;
console.log(length);
let func = function dummyFunc(a,b,c) {
return 'A B and C';
}
let {name, length:funcLen} = func;
console.log(name, funcLen);
It is something like what you have can be extracted with the same variable name
The destructuring assignment is a JavaScript expression that makes it possible to unpack values from arrays or properties from objects into distinct variables. Let's get the month values from an array using destructuring assignment
var [one, two, three] = ['orange', 'mango', 'banana'];
console.log(one); // "orange"
console.log(two); // "mango"
console.log(three); // "banana"
and you can get user properties of an object using destructuring assignment,
var {name, age} = {name: 'John', age: 32};
console.log(name); // John
console.log(age); // 32
The De-structured assignment of Javascript is probably an inspiration drawn from Perl language.
This facilitates reuse by avoid writing getter methods or wrapper functions.
One best example that I found very helpful in particular was on reusing functions that return more data than what is required.
If there is a function that returns a list or an array or a json, and we are interested in only the first item of the list or array or json,
then we can simply use the de-structured assignment instead of writing a new wrapper function to extract the interesting data item.
I am looking to push only the values of ObjArray into another array called Array.
var Array = [];
var ObjArray = [{item1 : 'Foo'}, {item2 : 'Bar'}];
// Push Values from ObjArray into Array
pushValues(ObjArray, Array);
The expected output would be Array having only ['Foo', 'Bar']
Thanks!
edit: Sorry. I am asking how to push all values from ObjArray into Array.
You can try with Array.prototype.map()
The map() method creates a new array with the results of calling a provided function on every element in the calling array.
And Object.values()
The Object.values() method returns an array of a given object's own enumerable property values.
As you have single value in the object use [0] to return that.
var ObjArray = [{item1 : 'Foo'}, {item2 : 'Bar'}];
var array = ObjArray.map(i => Object.values(i)[0]);
console.log(array);
Using for...of and push()
var array = [];
var ObjArray = [{item1 : 'Foo'}, {item2 : 'Bar'}];
for(var i of ObjArray){
array.push(Object.values(i)[0]);
}
console.log(array);
If your objects have multiple key-value pairs within them you can use a combination of .flatMap() with Object.values() like so:
const objArray = [{a: 'foo', b: 'bar'}, {c : 'baz'}];
const arr = objArray.flatMap(Object.values);
console.log(arr); // ["foo", "bar", "baz"]
However, do note, .flatMap() does have limited browser support, and so, if you need something a little more compatible, you can use .reduce() instead:
const objArray = [{a: 'foo', b: 'bar'}, {c : 'baz'}];
const arr = objArray.reduce((a, o) => [...a, ...Object.values(o)], []);
console.log(arr); // ["foo", "bar", "baz"]
I have been reading about Destructuring assignment introduced in ES6.
What is the purpose of this syntax, why was it introduced, and what are some examples of how it might be used in practice?
What is destructuring assignment ?
The destructuring assignment syntax is a JavaScript expression that makes it possible to unpack values from arrays, or properties from objects, into distinct variables.
- MDN
Advantages
A. Makes code concise and more readable.
B. We can easily avoid repeated destructing expression.
Some use cases
1. To get values in variable from Objects,array
let obj = { 'a': 1,'b': {'b1': '1.1'}}
let {a,b,b:{b1}} = obj
console.log('a--> ' + a, '\nb--> ', b, '\nb1---> ', b1)
let obj2 = { foo: 'foo' };
let { foo: newVarName } = obj2;
console.log(newVarName);
let arr = [1, 2, 3, 4, 5]
let [first, second, ...rest] = arr
console.log(first, '\n', second, '\n', rest)
// Nested extraction is possible too:
let obj3 = { foo: { bar: 'bar' } };
let { foo: { bar } } = obj3;
console.log(bar);
2. To combine an array at any place with another array.
let arr = [2,3,4,5]
let newArr = [0,1,...arr,6,7]
console.log(newArr)
3. To change only desired property in an object
let arr = [{a:1, b:2, c:3},{a:4, b:5, c:6},{a:7, b:8, c:9}]
let op = arr.map( ( {a,...rest}, index) => ({...rest,a:index+10}))
console.log(op)
4. To create a shallow copy of objects
let obj = {a:1,b:2,c:3}
let newObj = {...obj}
newObj.a = 'new Obj a'
console.log('Original Object', obj)
console.log('Shallow copied Object', newObj)
5. To extract values from parameters into standalone variables
// Object destructuring:
const fn = ({ prop }) => {
console.log(prop);
};
fn({ prop: 'foo' });
console.log('------------------');
// Array destructuring:
const fn2 = ([item1, item2]) => {
console.log(item1);
console.log(item2);
};
fn2(['bar', 'baz']);
console.log('------------------');
// Assigning default values to destructured properties:
const fn3 = ({ foo="defaultFooVal", bar }) => {
console.log(foo, bar);
};
fn3({ bar: 'bar' });
6. To get dynamic keys value from object
let obj = {a:1,b:2,c:3}
let key = 'c'
let {[key]:value} = obj
console.log(value)
7. To build an object from other object with some default values
let obj = {a:1,b:2,c:3}
let newObj = (({d=4,...rest} = obj), {d,...rest})
console.log(newObj)
8. To swap values
const b = [1, 2, 3, 4];
[b[0], b[2]] = [b[2], b[0]]; // swap index 0 and 2
console.log(b);
9. To get a subset of an object
9.1 subset of an object:
const obj = {a:1, b:2, c:3},
subset = (({a, c}) => ({a, c}))(obj); // credit to Ivan N for this function
console.log(subset);
9.2 To get a subset of an object using comma operator and destructuring:
const object = { a: 5, b: 6, c: 7 };
const picked = ({a,c}=object, {a,c})
console.log(picked); // { a: 5, c: 7 }
10. To do array to object conversion:
const arr = ["2019", "09", "02"],
date = (([year, day, month]) => ({year, month, day}))(arr);
console.log(date);
11. To set default values in function. (Read this answer for more info )
function someName(element, input,settings={i:"#1d252c", i2:"#fff",...input}){
console.log(settings.i)
console.log(settings.i2)
}
someName('hello', {i:'#123'})
someName('hello', {i2:'#123'})
12. To get properties such as length from an array, function name, number of arguments etc.
let arr = [1,2,3,4,5];
let {length} = arr;
console.log(length);
let func = function dummyFunc(a,b,c) {
return 'A B and C';
}
let {name, length:funcLen} = func;
console.log(name, funcLen);
It is something like what you have can be extracted with the same variable name
The destructuring assignment is a JavaScript expression that makes it possible to unpack values from arrays or properties from objects into distinct variables. Let's get the month values from an array using destructuring assignment
var [one, two, three] = ['orange', 'mango', 'banana'];
console.log(one); // "orange"
console.log(two); // "mango"
console.log(three); // "banana"
and you can get user properties of an object using destructuring assignment,
var {name, age} = {name: 'John', age: 32};
console.log(name); // John
console.log(age); // 32
The De-structured assignment of Javascript is probably an inspiration drawn from Perl language.
This facilitates reuse by avoid writing getter methods or wrapper functions.
One best example that I found very helpful in particular was on reusing functions that return more data than what is required.
If there is a function that returns a list or an array or a json, and we are interested in only the first item of the list or array or json,
then we can simply use the de-structured assignment instead of writing a new wrapper function to extract the interesting data item.
I have
var obj = {'b': 2, 'c': 3};
And i would like to add a property at the beginning (not at the end) of that object:
var obj = {'a': 1, 'b': 2, 'c': 3};
Is there a clean way to do that?
You can also use Object.assign() in ES6 (ES2015+).
let obj = {'b': 2, 'c': 3};
const returnedTarget = Object.assign({a: 1}, obj);
// Object {a: 1, b: 2, c: 3}
These days you could use the cool spread operator (...) in ES6 (ES2015+), try out the following:
const obj = {'b': 2, 'c': 3};
const startAdded = {'a':1 , ...obj};
console.log(startAdded);
const endAdded = {...obj, 'd':4};
console.log(endAdded);
Might help someone out there in the wild :)
The simplest way is to use the spread operator.
let obj = {'b': 2, 'c': 3};
let newObj = {'a': 1, ...obj};
JavaScript objects are unordered. There is no beginning or end. If you want order, use an array.
var arr = [
{ key: 'b', value: 2 },
{ key: 'c', value: 3 }
];
You can then add to the front of it with unshift:
arr.unshift({ key: 'a', value: 1 });
This Can be done using the lodash merge function like so:
var myObj = _.merge({ col1: 'col 1', col2: 'col 2'}, { col3: 'col 3', col4: 'col 4' });
Your final object will look like this:
{ col1: 'col 1', col2: 'col 2', col3: 'col 3', col4: 'col 4' }
As others mentioned, there is no guarantee that the order of the keys in the object will remain the same, depending on what you do with it. But, if you perform the merge as your final step, you should be ok. Note, the 'merge' function will produce a completely new object, it will not alter either of the objects you pass into it.
var obj = {'b': 2, 'c': 3};
obj = Object.assign({a: 1}, obj);
console.log(obj); // {a: 1, b: 2, c: 3}
Hope this helps
Javascript may not specify an order for properties in the language itself, but at least in my project (and perhaps yours), it certainly acts as if it does (JSON.stringify, debug watch windows, etc). I've never seen the for...in order be anything other than the order in which the properties were originally added. So for all intents and purposes in my project, and perhaps yours, this is very predictable.
And although this doesn't affect execution, this is a pain if you want to see a uniform list when properties of different items may be added in different order depending on how the object was created. Execution might not be the only thing that matters in project development. The ability to quickly visually inspect your objects also might matter.
Method 1 (high tech)
If you have spread, I would use that if you don't mind another object. But if you don't have spread (like in the older google apps script), or you need to keep the original object, you can do this:
objects.js
// Insert Property
Object.defineProperty(
Object.prototype,
'insertProperty',
{
value: function(name, value, index){
// backup my properties
var backup = {};
// delete all properties after index
var propertyIndex = 0;
for(var propertyName in this) {
if(this.hasOwnProperty(propertyName) && propertyIndex++ >= index) {
backup[propertyName] = this[propertyName];
delete this[propertyName];
}
}
this[name] = value;
// restore all properties after index
for(var propertyName in backup) {
if(backup.hasOwnProperty(propertyName))
this[propertyName] = backup[propertyName];
}
return this; // same object; not a new object
},
enumerable: false,
});
usage
var obj = {'b': 2, 'c': 3};
obj.insertProperty('a',1,0); // {a:1, b:2, c:3}
notes
You can use any index to put the property wherever you want it. 0 puts it at the front.
You can certainly pull the function out and add an obj parameter if you don't like adding methods to the object prototype. In my project, adding methods to the prototype helps me; it doesn't hurt me.
Method 2 (low tech)
In some cases, you might be able to just create your object with all the properties you know it may eventually have in the order you want to see them appear, and later, instead of inserting the property, just set the property.
// creation
var a = {Id: null, foo: 'bar'};
// update later
if(!a.Id)
a.Id = '123';
What worked for me is to use a temporary object for storing items in the desired order.
For example:
var new_object = {};
new_object[name] = value; // The property we need at the start
for (var key in old_object) { // Looping through all values of the old object
new_object[key] = old_object[key];
delete old_object[key];
}
old_object = new_object; // Replacing the old object with the desired one