Related
How do I check if a particular key exists in a JavaScript object or array?
If a key doesn't exist, and I try to access it, will it return false? Or throw an error?
Checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actually undefined?
var obj = { key: undefined };
console.log(obj["key"] !== undefined); // false, but the key exists!
You should instead use the in operator:
var obj = { key: undefined };
console.log("key" in obj); // true, regardless of the actual value
If you want to check if a key doesn't exist, remember to use parenthesis:
var obj = { not_key: undefined };
console.log(!("key" in obj)); // true if "key" doesn't exist in object
console.log(!"key" in obj); // Do not do this! It is equivalent to "false in obj"
Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty:
var obj = { key: undefined };
console.log(obj.hasOwnProperty("key")); // true
For performance comparison between the methods that are in, hasOwnProperty and key is undefined, see this benchmark:
Quick Answer
How do I check if a particular key exists in a JavaScript object or array?
If a key doesn't exist and I try to access it, will it return false? Or throw an error?
Accessing directly a missing property using (associative) array style or object style will return an undefined constant.
The slow and reliable in operator and hasOwnProperty method
As people have already mentioned here, you could have an object with a property associated with an "undefined" constant.
var bizzareObj = {valid_key: undefined};
In that case, you will have to use hasOwnProperty or in operator to know if the key is really there. But, but at what price?
so, I tell you...
in operator and hasOwnProperty are "methods" that use the Property Descriptor mechanism in Javascript (similar to Java reflection in the Java language).
http://www.ecma-international.org/ecma-262/5.1/#sec-8.10
The Property Descriptor type is used to explain the manipulation and reification of named property attributes. Values of the Property Descriptor type are records composed of named fields where each field’s name is an attribute name and its value is a corresponding attribute value as specified in 8.6.1. In addition, any field may be present or absent.
On the other hand, calling an object method or key will use Javascript [[Get]] mechanism. That is a far way faster!
Benchmark
https://jsben.ch/HaHQt
.
Using in operator
var result = "Impression" in array;
The result was
12,931,832 ±0.21% ops/sec 92% slower
Using hasOwnProperty
var result = array.hasOwnProperty("Impression")
The result was
16,021,758 ±0.45% ops/sec 91% slower
Accessing elements directly (brackets style)
var result = array["Impression"] === undefined
The result was
168,270,439 ±0.13 ops/sec 0.02% slower
Accessing elements directly (object style)
var result = array.Impression === undefined;
The result was
168,303,172 ±0.20% fastest
EDIT: What is the reason to assign to a property the undefined value?
That question puzzles me. In Javascript, there are at least two references for absent objects to avoid problems like this: null and undefined.
null is the primitive value that represents the intentional absence of any object value, or in short terms, the confirmed lack of value. On the other hand, undefined is an unknown value (not defined). If there is a property that will be used later with a proper value consider use null reference instead of undefined because in the initial moment the property is confirmed to lack value.
Compare:
var a = {1: null};
console.log(a[1] === undefined); // output: false. I know the value at position 1 of a[] is absent and this was by design, i.e.: the value is defined.
console.log(a[0] === undefined); // output: true. I cannot say anything about a[0] value. In this case, the key 0 was not in a[].
Advice
Avoid objects with undefined values. Check directly whenever possible and use null to initialize property values. Otherwise, use the slow in operator or hasOwnProperty() method.
EDIT: 12/04/2018 - NOT RELEVANT ANYMORE
As people have commented, modern versions of the Javascript engines (with firefox exception) have changed the approach for access properties. The current implementation is slower than the previous one for this particular case but the difference between access key and object is neglectable.
It will return undefined.
var aa = {hello: "world"};
alert( aa["hello"] ); // popup box with "world"
alert( aa["goodbye"] ); // popup box with "undefined"
undefined is a special constant value. So you can say, e.g.
// note the three equal signs so that null won't be equal to undefined
if( aa["goodbye"] === undefined ) {
// do something
}
This is probably the best way to check for missing keys. However, as is pointed out in a comment below, it's theoretically possible that you'd want to have the actual value be undefined. I've never needed to do this and can't think of a reason offhand why I'd ever want to, but just for the sake of completeness, you can use the in operator
// this works even if you have {"goodbye": undefined}
if( "goodbye" in aa ) {
// do something
}
"key" in obj
Is likely testing only object attribute values that are very different from array keys
Checking for properties of the object including inherited properties
Could be determined using the in operator which returns true if the specified property is in the specified object or its prototype chain, false otherwise
const person = { name: 'dan' };
console.log('name' in person); // true
console.log('age' in person); // false
Checking for properties of the object instance (not including inherited properties)
*2021 - Using the new method ***Object.hasOwn() as a replacement for Object.hasOwnProperty()
Object.hasOwn() is intended as a replacement for Object.hasOwnProperty() and is a new method available to use (yet still not fully supported by all browsers like safari yet but soon will be)
Object.hasOwn() is a static method which returns true if the specified object has the specified property as its own property. If the property is inherited, or does not exist, the method returns false.
const person = { name: 'dan' };
console.log(Object.hasOwn(person, 'name'));// true
console.log(Object.hasOwn(person, 'age'));// false
const person2 = Object.create({gender: 'male'});
console.log(Object.hasOwn(person2, 'gender'));// false
What is the motivation to use it over Object.prototype.hasOwnProperty? - It is recommended to this method use over the Object.hasOwnProperty() because it also works for objects created by using Object.create(null) and for objects that have overridden the inherited hasOwnProperty() method. Although it's possible to solve these kind of problems by calling Object.prototype.hasOwnProperty() on an external object, Object.hasOwn() overcome these problems, hence is preferred (see examples below)
let person = {
hasOwnProperty: function() {
return false;
},
age: 35
};
if (Object.hasOwn(person, 'age')) {
console.log(person.age); // true - the remplementation of hasOwnProperty() did not affect the Object
}
let person = Object.create(null);
person.age = 35;
if (Object.hasOwn(person, 'age')) {
console.log(person.age); // true - works regardless of how the object was created
}
More about Object.hasOwn can be found here : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn
Browser compatibility for Object.hasOwn - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn#browser_compatibility
The accepted answer refers to Object. Beware using the in operator on Array to find data instead of keys:
("true" in ["true", "false"])
// -> false (Because the keys of the above Array are actually 0 and 1)
To test existing elements in an Array: Best way to find if an item is in a JavaScript array?
Three ways to check if a property is present in a javascript object:
!!obj.theProperty
Will convert value to bool. returns true for all but the false value
'theProperty' in obj
Will return true if the property exists, no matter its value (even empty)
obj.hasOwnProperty('theProperty')
Does not check the prototype chain. (since all objects have the toString method, 1 and 2 will return true on it, while 3 can return false on it.)
Reference:
http://book.mixu.net/node/ch5.html
If you are using underscore.js library then object/array operations become simple.
In your case _.has method can be used. Example:
yourArray = {age: "10"}
_.has(yourArray, "age")
returns true
But,
_.has(yourArray, "invalidKey")
returns false
Answer:
if ("key" in myObj)
{
console.log("key exists!");
}
else
{
console.log("key doesn't exist!");
}
Explanation:
The in operator will check if the key exists in the object. If you checked if the value was undefined: if (myObj["key"] === 'undefined'), you could run into problems because a key could possibly exist in your object with the undefined value.
For that reason, it is much better practice to first use the in operator and then compare the value that is inside the key once you already know it exists.
Here's a helper function I find quite useful
This keyExists(key, search) can be used to easily lookup a key within objects or arrays!
Just pass it the key you want to find, and search obj (the object or array) you want to find it in.
function keyExists(key, search) {
if (!search || (search.constructor !== Array && search.constructor !== Object)) {
return false;
}
for (var i = 0; i < search.length; i++) {
if (search[i] === key) {
return true;
}
}
return key in search;
}
// How to use it:
// Searching for keys in Arrays
console.log(keyExists('apple', ['apple', 'banana', 'orange'])); // true
console.log(keyExists('fruit', ['apple', 'banana', 'orange'])); // false
// Searching for keys in Objects
console.log(keyExists('age', {'name': 'Bill', 'age': 29 })); // true
console.log(keyExists('title', {'name': 'Jason', 'age': 29 })); // false
It's been pretty reliable and works well cross-browser.
vanila js
yourObjName.hasOwnProperty(key) : true ? false;
If you want to check if the object has at least one property in es2015
Object.keys(yourObjName).length : true ? false
ES6 solution
using Array#some and Object.keys. It will return true if given key exists in the object or false if it doesn't.
var obj = {foo: 'one', bar: 'two'};
function isKeyInObject(obj, key) {
var res = Object.keys(obj).some(v => v == key);
console.log(res);
}
isKeyInObject(obj, 'foo');
isKeyInObject(obj, 'something');
One-line example.
console.log(Object.keys({foo: 'one', bar: 'two'}).some(v => v == 'foo'));
Optional chaining operator:
const invoice = {customer: {address: {city: "foo"}}}
console.log( invoice?.customer?.address?.city )
console.log( invoice?.customer?.address?.street )
console.log( invoice?.xyz?.address?.city )
See supported browsers list
For those which have lodash included in their project:There is a lodash _.get method which tries to get "deep" keys:
Gets the value at path of object. If the resolved value is undefined,
the defaultValue is returned in its place.
var object = { 'a': [{ 'b': { 'c': 3 } }] };
console.log(
_.get(object, 'a[0].b.c'), // => 3
_.get(object, ['a', '0', 'b', 'c']), // => 3
_.get(object, 'a.b.c'), // => undefined
_.get(object, 'a.b.c', 'default') // => 'default'
)
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.4/lodash.min.js"></script>
This will effectively check if that key, however deep, is defined and will not throw an error which might harm the flow of your program if that key is not defined.
To find if a key exists in an object, use
Object.keys(obj).includes(key)
The ES7 includes method checks if an Array includes an item or not, & is a simpler alternative to indexOf.
The easiest way to check is
"key" in object
for example:
var obj = {
a: 1,
b: 2,
}
"a" in obj // true
"c" in obj // false
Return value as true implies that key exists in the object.
Optional Chaining (?.) operator can also be used for this
Source: MDN/Operators/Optional_chaining
const adventurer = {
name: 'Alice',
cat: {
name: 'Dinah'
}
}
console.log(adventurer.dog?.name) // undefined
console.log(adventurer.cat?.name) // Dinah
An alternate approach using "Reflect"
As per MDN
Reflect is a built-in object that provides methods for interceptable
JavaScript operations.
The static Reflect.has() method works like the in operator as a
function.
var obj = {
a: undefined,
b: 1,
c: "hello world"
}
console.log(Reflect.has(obj, 'a'))
console.log(Reflect.has(obj, 'b'))
console.log(Reflect.has(obj, 'c'))
console.log(Reflect.has(obj, 'd'))
Should I use it ?
It depends.
Reflect.has() is slower than the other methods mentioned on the accepted answer (as per my benchmark test). But, if you are using it only a few times in your code, I don't see much issues with this approach.
We can use - hasOwnProperty.call(obj, key);
The underscore.js way -
if(_.has(this.options, 'login')){
//key 'login' exists in this.options
}
_.has = function(obj, key) {
return hasOwnProperty.call(obj, key);
};
If you want to check for any key at any depth on an object and account for falsey values consider this line for a utility function:
var keyExistsOn = (o, k) => k.split(".").reduce((a, c) => a.hasOwnProperty(c) ? a[c] || 1 : false, Object.assign({}, o)) === false ? false : true;
Results
var obj = {
test: "",
locals: {
test: "",
test2: false,
test3: NaN,
test4: 0,
test5: undefined,
auth: {
user: "hw"
}
}
}
keyExistsOn(obj, "")
> false
keyExistsOn(obj, "locals.test")
> true
keyExistsOn(obj, "locals.test2")
> true
keyExistsOn(obj, "locals.test3")
> true
keyExistsOn(obj, "locals.test4")
> true
keyExistsOn(obj, "locals.test5")
> true
keyExistsOn(obj, "sdsdf")
false
keyExistsOn(obj, "sdsdf.rtsd")
false
keyExistsOn(obj, "sdsdf.234d")
false
keyExistsOn(obj, "2134.sdsdf.234d")
false
keyExistsOn(obj, "locals")
true
keyExistsOn(obj, "locals.")
false
keyExistsOn(obj, "locals.auth")
true
keyExistsOn(obj, "locals.autht")
false
keyExistsOn(obj, "locals.auth.")
false
keyExistsOn(obj, "locals.auth.user")
true
keyExistsOn(obj, "locals.auth.userr")
false
keyExistsOn(obj, "locals.auth.user.")
false
keyExistsOn(obj, "locals.auth.user")
true
Also see this NPM package: https://www.npmjs.com/package/has-deep-value
While this doesn't necessarily check if a key exists, it does check for the truthiness of a value. Which undefined and null fall under.
Boolean(obj.foo)
This solution works best for me because I use typescript, and using strings like so 'foo' in obj or obj.hasOwnProperty('foo')
to check whether a key exists or not does not provide me with intellisense.
const object1 = {
a: 'something',
b: 'something',
c: 'something'
};
const key = 's';
// Object.keys(object1) will return array of the object keys ['a', 'b', 'c']
Object.keys(object1).indexOf(key) === -1 ? 'the key is not there' : 'yep the key is exist';
In 'array' world we can look on indexes as some kind of keys. What is surprising the in operator (which is good choice for object) also works with arrays. The returned value for non-existed key is undefined
let arr = ["a","b","c"]; // we have indexes: 0,1,2
delete arr[1]; // set 'empty' at index 1
arr.pop(); // remove last item
console.log(0 in arr, arr[0]);
console.log(1 in arr, arr[1]);
console.log(2 in arr, arr[2]);
Worth noting that since the introduction of ES11 you can use the nullish coalescing operator, which simplifies things a lot:
const obj = {foo: 'one', bar: 'two'};
const result = obj.foo ?? "Not found";
The code above will return "Not found" for any "falsy" values in foo. Otherwise it will return obj.foo.
See Combining with the nullish coalescing operator
JS Double Exclamation !! sign may help in this case.
const cars = {
petrol:{
price: 5000
},
gas:{
price:8000
}
}
Suppose we have the object above and If you try to log car with petrol price.
=> console.log(cars.petrol.price);
=> 5000
You'll definitely get 5000 out of it. But what if you try to get an
electric car which does not exist then you'll get undefine
=> console.log(cars.electric);
=> undefine
But using !! which is its short way to cast a variable to be a
Boolean (true or false) value.
=> console.log(!!cars.electric);
=> false
In my case, I wanted to check an NLP metadata returned by LUIS which is an object. I wanted to check if a key which is a string "FinancialRiskIntent" exists as a key inside that metadata object.
I tried to target the nested object I needed to check -> data.meta.prediction.intents (for my own purposes only, yours could be any object)
I used below code to check if the key exists:
const hasKey = 'FinancialRiskIntent' in data.meta.prediction.intents;
if(hasKey) {
console.log('The key exists.');
}
else {
console.log('The key does not exist.');
}
This is checking for a specific key which I was initially looking for.
Hope this bit helps someone.
yourArray.indexOf(yourArrayKeyName) > -1
fruit = ['apple', 'grapes', 'banana']
fruit.indexOf('apple') > -1
true
fruit = ['apple', 'grapes', 'banana']
fruit.indexOf('apple1') > -1
false
for strict object keys checking:
const object1 = {};
object1.stackoverflow = 51;
console.log(object1.hasOwnProperty('stackoverflow'));
output: true
These example can demonstrate the differences between defferent ways. Hope it will help you to pick the right one for your needs:
// Lets create object `a` using create function `A`
function A(){};
A.prototype.onProtDef=2;
A.prototype.onProtUndef=undefined;
var a=new A();
a.ownProp = 3;
a.ownPropUndef = undefined;
// Let's try different methods:
a.onProtDef; // 2
a.onProtUndef; // undefined
a.ownProp; // 3
a.ownPropUndef; // undefined
a.whatEver; // undefined
a.valueOf; // ƒ valueOf() { [native code] }
a.hasOwnProperty('onProtDef'); // false
a.hasOwnProperty('onProtUndef'); // false
a.hasOwnProperty('ownProp'); // true
a.hasOwnProperty('ownPropUndef'); // true
a.hasOwnProperty('whatEver'); // false
a.hasOwnProperty('valueOf'); // false
'onProtDef' in a; // true
'onProtUndef' in a; // true
'ownProp' in a; // true
'ownPropUndef' in a; // true
'whatEver' in a; // false
'valueOf' in a; // true (on the prototype chain - Object.valueOf)
Object.keys(a); // ["ownProp", "ownPropUndef"]
const rawObject = {};
rawObject.propertyKey = 'somethingValue';
console.log(rawObject.hasOwnProperty('somethingValue'));
// expected output: true
checking particular key present in given object, hasOwnProperty will works here.
If you have ESLint configured in your project follows ESLint rule no-prototype-builtins. The reason why has been described in the following link:
// bad
console.log(object.hasOwnProperty(key));
// good
console.log(Object.prototype.hasOwnProperty.call(object, key));
// best
const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope.
console.log(has.call(object, key));
/* or */
import has from 'has'; // https://www.npmjs.com/package/has
console.log(has(object, key));
New awesome solution with JavaScript Destructuring:
let obj = {
"key1": "value1",
"key2": "value2",
"key3": "value3",
};
let {key1, key2, key3, key4} = obj;
// key1 = "value1"
// key2 = "value2"
// key3 = "value3"
// key4 = undefined
// Can easily use `if` here on key4
if(!key4) { console.log("key not present"); } // Key not present
Do check other use of JavaScript Destructuring
This question already has answers here:
How do I test for an empty JavaScript object?
(48 answers)
Closed 7 months ago.
I'm learing JavaScript. I cannot grasp the idea of an empty object. As I understand, there are situations when I need to check a variable whether it holds an object and has a value.
So far, I know that a variable can be undefined.
var car; // the value is undefined, as well as the type (which is it? number, array, etc.)
I also know, that everything that has a value, is true:
var car = "Opel";
Boolean(car); // true
And anything without a value is false:
var car = ""; // typeof string, but it is empty, so
Boolean(car); // false
Boolean(null); // false
So why doesn't it work the same way with arrays and objects? Why is all of this true? Shouldn't empty arrays and objects return false?
var car = {make: "Opel", year: 2015};
Boolean(car); // true
car = {};
Boolean(car); // true
car = ["BMW", "Opel"];
Boolean(car); // true
car = [];
Boolean(car); // true
Now I see that there are methods, that can be applied to check an object's length, I just haven't reached that part yet.
I'm learning at W3Schools website and this bit just got me puzzled:
But you cannot test if an object is null, because this will throw an error if the object is undefined:
Incorrect:
if (myObj === null)
To solve this problem, you must test if an object is not null, and not undefined.
But this can still throw an error:
Incorrect:
if (myObj !== null && typeof myObj !== "undefined")
Because of this, you must test for not undefined before you can test for not null:
Correct:
if (typeof myObj !== "undefined" && myObj !== null)
I still cannot understand the last line here.
Checking if an object is empty:
Object.keys(yourObject).length
var car = {};
var isEmpty = Object.entries(car).length > 0; //false
var car = {make: "Opel", year: 2015};
var isEmpty = Object.entries(car).length > 0; //true
This should solve your problem, if your curious about the utility function Object.entries you can look on mdn
I also know, that everything that has a value, is true
I wouldn't say that. All the examples given are values, even undefined and null are predefined values in JavaScript.
Shouldn't empty arrays and objects return false?
It is like that by specification. And that is really the answer to your question. It is a choice made by the designers of the language.
You could find some logic in it though. In JavaScript, values representing objects (and so also arrays) are references. These references are memory addresses/pointers, and although you cannot find out what exactly those addresses are, these are non-zero, even when an object has no properties.
Note that your example objects ({} and []) still have some non-enumerable properties, such a __proto__ and (for array) length, so they are not really as empty as you think.
The most empty object you can create, is: Object.create(null)
You can check wheather a object is empty or not in this simple way.
function objectLength( object ) {
return Object.keys(object).length;
}
if(objectLength(yourObject) == 0){
//object is empty
}else{
//object not empty
}
Checking if an object is empty:
Reflect.ownKeys(car).length
Returns an array with one element when a Symbol is used as the key:
let key = Symbol('vin')
let car = { [key]: 'honda' }
Reflect.ownKeys(car).length // => 1
Whereas Object.keys returns an array with zero elements in this case:
let key = Symbol('vin')
let car = { [key]: 'honda' }
Object.keys(car).length // => 0
I would always use typeof myvar !== 'undefined' and checks for content myvar !== ''
This would always would have a clear result.
There are concepts which are Truthy and Falsy values in JavaScript.
I highly recommend you to read MDN documents about this issue.
All values are truthy unless they are defined as falsy (i.e., except for false, 0, -0, 0n, "", null, undefined, and NaN).
Truthy values
Falsy values
No, they should not be false.
All objects are 'truthy', in that they return true when evaluated as a Boolean. In Javascript, all arrays are objects (try console.log(typeof [])), so they also return true, regardless of whether or not they are empty.
To check if any array is empty:
if (MYARRAY.length === 0) {
// console.log(true);
}
To check if an object is empty:
if (Object.keys(MYOBJECT).length === 0) {
// console.log(true);
}
You can use
if ($('#element').is(':empty')){
//do something
}
OR
function isEmpty( el ){
return !$.trim(el.html())
}
if (isEmpty($('#element'))) {
// do something
}
Here are some examples: http://api.jquery.com/is/
But if you need it in JavaScript:
if( typeof foo !== 'undefined' ) {
// foo could get resolved and it's defined
}
You can simply use typeof
How do I check if a particular key exists in a JavaScript object or array?
If a key doesn't exist, and I try to access it, will it return false? Or throw an error?
Checking for undefined-ness is not an accurate way of testing whether a key exists. What if the key exists but the value is actually undefined?
var obj = { key: undefined };
console.log(obj["key"] !== undefined); // false, but the key exists!
You should instead use the in operator:
var obj = { key: undefined };
console.log("key" in obj); // true, regardless of the actual value
If you want to check if a key doesn't exist, remember to use parenthesis:
var obj = { not_key: undefined };
console.log(!("key" in obj)); // true if "key" doesn't exist in object
console.log(!"key" in obj); // Do not do this! It is equivalent to "false in obj"
Or, if you want to particularly test for properties of the object instance (and not inherited properties), use hasOwnProperty:
var obj = { key: undefined };
console.log(obj.hasOwnProperty("key")); // true
For performance comparison between the methods that are in, hasOwnProperty and key is undefined, see this benchmark:
Quick Answer
How do I check if a particular key exists in a JavaScript object or array?
If a key doesn't exist and I try to access it, will it return false? Or throw an error?
Accessing directly a missing property using (associative) array style or object style will return an undefined constant.
The slow and reliable in operator and hasOwnProperty method
As people have already mentioned here, you could have an object with a property associated with an "undefined" constant.
var bizzareObj = {valid_key: undefined};
In that case, you will have to use hasOwnProperty or in operator to know if the key is really there. But, but at what price?
so, I tell you...
in operator and hasOwnProperty are "methods" that use the Property Descriptor mechanism in Javascript (similar to Java reflection in the Java language).
http://www.ecma-international.org/ecma-262/5.1/#sec-8.10
The Property Descriptor type is used to explain the manipulation and reification of named property attributes. Values of the Property Descriptor type are records composed of named fields where each field’s name is an attribute name and its value is a corresponding attribute value as specified in 8.6.1. In addition, any field may be present or absent.
On the other hand, calling an object method or key will use Javascript [[Get]] mechanism. That is a far way faster!
Benchmark
https://jsben.ch/HaHQt
.
Using in operator
var result = "Impression" in array;
The result was
12,931,832 ±0.21% ops/sec 92% slower
Using hasOwnProperty
var result = array.hasOwnProperty("Impression")
The result was
16,021,758 ±0.45% ops/sec 91% slower
Accessing elements directly (brackets style)
var result = array["Impression"] === undefined
The result was
168,270,439 ±0.13 ops/sec 0.02% slower
Accessing elements directly (object style)
var result = array.Impression === undefined;
The result was
168,303,172 ±0.20% fastest
EDIT: What is the reason to assign to a property the undefined value?
That question puzzles me. In Javascript, there are at least two references for absent objects to avoid problems like this: null and undefined.
null is the primitive value that represents the intentional absence of any object value, or in short terms, the confirmed lack of value. On the other hand, undefined is an unknown value (not defined). If there is a property that will be used later with a proper value consider use null reference instead of undefined because in the initial moment the property is confirmed to lack value.
Compare:
var a = {1: null};
console.log(a[1] === undefined); // output: false. I know the value at position 1 of a[] is absent and this was by design, i.e.: the value is defined.
console.log(a[0] === undefined); // output: true. I cannot say anything about a[0] value. In this case, the key 0 was not in a[].
Advice
Avoid objects with undefined values. Check directly whenever possible and use null to initialize property values. Otherwise, use the slow in operator or hasOwnProperty() method.
EDIT: 12/04/2018 - NOT RELEVANT ANYMORE
As people have commented, modern versions of the Javascript engines (with firefox exception) have changed the approach for access properties. The current implementation is slower than the previous one for this particular case but the difference between access key and object is neglectable.
It will return undefined.
var aa = {hello: "world"};
alert( aa["hello"] ); // popup box with "world"
alert( aa["goodbye"] ); // popup box with "undefined"
undefined is a special constant value. So you can say, e.g.
// note the three equal signs so that null won't be equal to undefined
if( aa["goodbye"] === undefined ) {
// do something
}
This is probably the best way to check for missing keys. However, as is pointed out in a comment below, it's theoretically possible that you'd want to have the actual value be undefined. I've never needed to do this and can't think of a reason offhand why I'd ever want to, but just for the sake of completeness, you can use the in operator
// this works even if you have {"goodbye": undefined}
if( "goodbye" in aa ) {
// do something
}
"key" in obj
Is likely testing only object attribute values that are very different from array keys
Checking for properties of the object including inherited properties
Could be determined using the in operator which returns true if the specified property is in the specified object or its prototype chain, false otherwise
const person = { name: 'dan' };
console.log('name' in person); // true
console.log('age' in person); // false
Checking for properties of the object instance (not including inherited properties)
*2021 - Using the new method ***Object.hasOwn() as a replacement for Object.hasOwnProperty()
Object.hasOwn() is intended as a replacement for Object.hasOwnProperty() and is a new method available to use (yet still not fully supported by all browsers like safari yet but soon will be)
Object.hasOwn() is a static method which returns true if the specified object has the specified property as its own property. If the property is inherited, or does not exist, the method returns false.
const person = { name: 'dan' };
console.log(Object.hasOwn(person, 'name'));// true
console.log(Object.hasOwn(person, 'age'));// false
const person2 = Object.create({gender: 'male'});
console.log(Object.hasOwn(person2, 'gender'));// false
What is the motivation to use it over Object.prototype.hasOwnProperty? - It is recommended to this method use over the Object.hasOwnProperty() because it also works for objects created by using Object.create(null) and for objects that have overridden the inherited hasOwnProperty() method. Although it's possible to solve these kind of problems by calling Object.prototype.hasOwnProperty() on an external object, Object.hasOwn() overcome these problems, hence is preferred (see examples below)
let person = {
hasOwnProperty: function() {
return false;
},
age: 35
};
if (Object.hasOwn(person, 'age')) {
console.log(person.age); // true - the remplementation of hasOwnProperty() did not affect the Object
}
let person = Object.create(null);
person.age = 35;
if (Object.hasOwn(person, 'age')) {
console.log(person.age); // true - works regardless of how the object was created
}
More about Object.hasOwn can be found here : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn
Browser compatibility for Object.hasOwn - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwn#browser_compatibility
The accepted answer refers to Object. Beware using the in operator on Array to find data instead of keys:
("true" in ["true", "false"])
// -> false (Because the keys of the above Array are actually 0 and 1)
To test existing elements in an Array: Best way to find if an item is in a JavaScript array?
Three ways to check if a property is present in a javascript object:
!!obj.theProperty
Will convert value to bool. returns true for all but the false value
'theProperty' in obj
Will return true if the property exists, no matter its value (even empty)
obj.hasOwnProperty('theProperty')
Does not check the prototype chain. (since all objects have the toString method, 1 and 2 will return true on it, while 3 can return false on it.)
Reference:
http://book.mixu.net/node/ch5.html
If you are using underscore.js library then object/array operations become simple.
In your case _.has method can be used. Example:
yourArray = {age: "10"}
_.has(yourArray, "age")
returns true
But,
_.has(yourArray, "invalidKey")
returns false
Answer:
if ("key" in myObj)
{
console.log("key exists!");
}
else
{
console.log("key doesn't exist!");
}
Explanation:
The in operator will check if the key exists in the object. If you checked if the value was undefined: if (myObj["key"] === 'undefined'), you could run into problems because a key could possibly exist in your object with the undefined value.
For that reason, it is much better practice to first use the in operator and then compare the value that is inside the key once you already know it exists.
Here's a helper function I find quite useful
This keyExists(key, search) can be used to easily lookup a key within objects or arrays!
Just pass it the key you want to find, and search obj (the object or array) you want to find it in.
function keyExists(key, search) {
if (!search || (search.constructor !== Array && search.constructor !== Object)) {
return false;
}
for (var i = 0; i < search.length; i++) {
if (search[i] === key) {
return true;
}
}
return key in search;
}
// How to use it:
// Searching for keys in Arrays
console.log(keyExists('apple', ['apple', 'banana', 'orange'])); // true
console.log(keyExists('fruit', ['apple', 'banana', 'orange'])); // false
// Searching for keys in Objects
console.log(keyExists('age', {'name': 'Bill', 'age': 29 })); // true
console.log(keyExists('title', {'name': 'Jason', 'age': 29 })); // false
It's been pretty reliable and works well cross-browser.
vanila js
yourObjName.hasOwnProperty(key) : true ? false;
If you want to check if the object has at least one property in es2015
Object.keys(yourObjName).length : true ? false
ES6 solution
using Array#some and Object.keys. It will return true if given key exists in the object or false if it doesn't.
var obj = {foo: 'one', bar: 'two'};
function isKeyInObject(obj, key) {
var res = Object.keys(obj).some(v => v == key);
console.log(res);
}
isKeyInObject(obj, 'foo');
isKeyInObject(obj, 'something');
One-line example.
console.log(Object.keys({foo: 'one', bar: 'two'}).some(v => v == 'foo'));
Optional chaining operator:
const invoice = {customer: {address: {city: "foo"}}}
console.log( invoice?.customer?.address?.city )
console.log( invoice?.customer?.address?.street )
console.log( invoice?.xyz?.address?.city )
See supported browsers list
For those which have lodash included in their project:There is a lodash _.get method which tries to get "deep" keys:
Gets the value at path of object. If the resolved value is undefined,
the defaultValue is returned in its place.
var object = { 'a': [{ 'b': { 'c': 3 } }] };
console.log(
_.get(object, 'a[0].b.c'), // => 3
_.get(object, ['a', '0', 'b', 'c']), // => 3
_.get(object, 'a.b.c'), // => undefined
_.get(object, 'a.b.c', 'default') // => 'default'
)
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.4/lodash.min.js"></script>
This will effectively check if that key, however deep, is defined and will not throw an error which might harm the flow of your program if that key is not defined.
To find if a key exists in an object, use
Object.keys(obj).includes(key)
The ES7 includes method checks if an Array includes an item or not, & is a simpler alternative to indexOf.
The easiest way to check is
"key" in object
for example:
var obj = {
a: 1,
b: 2,
}
"a" in obj // true
"c" in obj // false
Return value as true implies that key exists in the object.
Optional Chaining (?.) operator can also be used for this
Source: MDN/Operators/Optional_chaining
const adventurer = {
name: 'Alice',
cat: {
name: 'Dinah'
}
}
console.log(adventurer.dog?.name) // undefined
console.log(adventurer.cat?.name) // Dinah
An alternate approach using "Reflect"
As per MDN
Reflect is a built-in object that provides methods for interceptable
JavaScript operations.
The static Reflect.has() method works like the in operator as a
function.
var obj = {
a: undefined,
b: 1,
c: "hello world"
}
console.log(Reflect.has(obj, 'a'))
console.log(Reflect.has(obj, 'b'))
console.log(Reflect.has(obj, 'c'))
console.log(Reflect.has(obj, 'd'))
Should I use it ?
It depends.
Reflect.has() is slower than the other methods mentioned on the accepted answer (as per my benchmark test). But, if you are using it only a few times in your code, I don't see much issues with this approach.
We can use - hasOwnProperty.call(obj, key);
The underscore.js way -
if(_.has(this.options, 'login')){
//key 'login' exists in this.options
}
_.has = function(obj, key) {
return hasOwnProperty.call(obj, key);
};
If you want to check for any key at any depth on an object and account for falsey values consider this line for a utility function:
var keyExistsOn = (o, k) => k.split(".").reduce((a, c) => a.hasOwnProperty(c) ? a[c] || 1 : false, Object.assign({}, o)) === false ? false : true;
Results
var obj = {
test: "",
locals: {
test: "",
test2: false,
test3: NaN,
test4: 0,
test5: undefined,
auth: {
user: "hw"
}
}
}
keyExistsOn(obj, "")
> false
keyExistsOn(obj, "locals.test")
> true
keyExistsOn(obj, "locals.test2")
> true
keyExistsOn(obj, "locals.test3")
> true
keyExistsOn(obj, "locals.test4")
> true
keyExistsOn(obj, "locals.test5")
> true
keyExistsOn(obj, "sdsdf")
false
keyExistsOn(obj, "sdsdf.rtsd")
false
keyExistsOn(obj, "sdsdf.234d")
false
keyExistsOn(obj, "2134.sdsdf.234d")
false
keyExistsOn(obj, "locals")
true
keyExistsOn(obj, "locals.")
false
keyExistsOn(obj, "locals.auth")
true
keyExistsOn(obj, "locals.autht")
false
keyExistsOn(obj, "locals.auth.")
false
keyExistsOn(obj, "locals.auth.user")
true
keyExistsOn(obj, "locals.auth.userr")
false
keyExistsOn(obj, "locals.auth.user.")
false
keyExistsOn(obj, "locals.auth.user")
true
Also see this NPM package: https://www.npmjs.com/package/has-deep-value
While this doesn't necessarily check if a key exists, it does check for the truthiness of a value. Which undefined and null fall under.
Boolean(obj.foo)
This solution works best for me because I use typescript, and using strings like so 'foo' in obj or obj.hasOwnProperty('foo')
to check whether a key exists or not does not provide me with intellisense.
const object1 = {
a: 'something',
b: 'something',
c: 'something'
};
const key = 's';
// Object.keys(object1) will return array of the object keys ['a', 'b', 'c']
Object.keys(object1).indexOf(key) === -1 ? 'the key is not there' : 'yep the key is exist';
In 'array' world we can look on indexes as some kind of keys. What is surprising the in operator (which is good choice for object) also works with arrays. The returned value for non-existed key is undefined
let arr = ["a","b","c"]; // we have indexes: 0,1,2
delete arr[1]; // set 'empty' at index 1
arr.pop(); // remove last item
console.log(0 in arr, arr[0]);
console.log(1 in arr, arr[1]);
console.log(2 in arr, arr[2]);
Worth noting that since the introduction of ES11 you can use the nullish coalescing operator, which simplifies things a lot:
const obj = {foo: 'one', bar: 'two'};
const result = obj.foo ?? "Not found";
The code above will return "Not found" for any "falsy" values in foo. Otherwise it will return obj.foo.
See Combining with the nullish coalescing operator
JS Double Exclamation !! sign may help in this case.
const cars = {
petrol:{
price: 5000
},
gas:{
price:8000
}
}
Suppose we have the object above and If you try to log car with petrol price.
=> console.log(cars.petrol.price);
=> 5000
You'll definitely get 5000 out of it. But what if you try to get an
electric car which does not exist then you'll get undefine
=> console.log(cars.electric);
=> undefine
But using !! which is its short way to cast a variable to be a
Boolean (true or false) value.
=> console.log(!!cars.electric);
=> false
In my case, I wanted to check an NLP metadata returned by LUIS which is an object. I wanted to check if a key which is a string "FinancialRiskIntent" exists as a key inside that metadata object.
I tried to target the nested object I needed to check -> data.meta.prediction.intents (for my own purposes only, yours could be any object)
I used below code to check if the key exists:
const hasKey = 'FinancialRiskIntent' in data.meta.prediction.intents;
if(hasKey) {
console.log('The key exists.');
}
else {
console.log('The key does not exist.');
}
This is checking for a specific key which I was initially looking for.
Hope this bit helps someone.
yourArray.indexOf(yourArrayKeyName) > -1
fruit = ['apple', 'grapes', 'banana']
fruit.indexOf('apple') > -1
true
fruit = ['apple', 'grapes', 'banana']
fruit.indexOf('apple1') > -1
false
for strict object keys checking:
const object1 = {};
object1.stackoverflow = 51;
console.log(object1.hasOwnProperty('stackoverflow'));
output: true
These example can demonstrate the differences between defferent ways. Hope it will help you to pick the right one for your needs:
// Lets create object `a` using create function `A`
function A(){};
A.prototype.onProtDef=2;
A.prototype.onProtUndef=undefined;
var a=new A();
a.ownProp = 3;
a.ownPropUndef = undefined;
// Let's try different methods:
a.onProtDef; // 2
a.onProtUndef; // undefined
a.ownProp; // 3
a.ownPropUndef; // undefined
a.whatEver; // undefined
a.valueOf; // ƒ valueOf() { [native code] }
a.hasOwnProperty('onProtDef'); // false
a.hasOwnProperty('onProtUndef'); // false
a.hasOwnProperty('ownProp'); // true
a.hasOwnProperty('ownPropUndef'); // true
a.hasOwnProperty('whatEver'); // false
a.hasOwnProperty('valueOf'); // false
'onProtDef' in a; // true
'onProtUndef' in a; // true
'ownProp' in a; // true
'ownPropUndef' in a; // true
'whatEver' in a; // false
'valueOf' in a; // true (on the prototype chain - Object.valueOf)
Object.keys(a); // ["ownProp", "ownPropUndef"]
const rawObject = {};
rawObject.propertyKey = 'somethingValue';
console.log(rawObject.hasOwnProperty('somethingValue'));
// expected output: true
checking particular key present in given object, hasOwnProperty will works here.
If you have ESLint configured in your project follows ESLint rule no-prototype-builtins. The reason why has been described in the following link:
// bad
console.log(object.hasOwnProperty(key));
// good
console.log(Object.prototype.hasOwnProperty.call(object, key));
// best
const has = Object.prototype.hasOwnProperty; // cache the lookup once, in module scope.
console.log(has.call(object, key));
/* or */
import has from 'has'; // https://www.npmjs.com/package/has
console.log(has(object, key));
New awesome solution with JavaScript Destructuring:
let obj = {
"key1": "value1",
"key2": "value2",
"key3": "value3",
};
let {key1, key2, key3, key4} = obj;
// key1 = "value1"
// key2 = "value2"
// key3 = "value3"
// key4 = undefined
// Can easily use `if` here on key4
if(!key4) { console.log("key not present"); } // Key not present
Do check other use of JavaScript Destructuring
I am very interested in understanding JavaScript behavior.
Object :
object = {object : window.object};
object = {object : window.object};
console.log(object === object); // true
console.log(object.object === object); // false
console.log(object.object === object.object); // true
console.log(object.object.object === object.object); // false
Array :
array = [window.array];
array = [window.array];
console.log(array === array); // true
console.log(array[0] === array); // false
console.log(array[0] === array[0]) // true
console.log(array[0][0] === array[0]) // false
Why
object.object.object === object.object
returns false ???
After the first assignment, you have the following:
object = { object: undefined }
The second assignment creates a new object, whose object property contains the previous value of window.object, and assigns this to object. So now you have:
object = { object: { object: undefined } }
So at this time, object.object is { object: undefined }, which is not the same as object.
Similar things happen with the array example.
If you want to create a self-referential object, you need to do:
object = {};
object.object = object;
This doesn't create a new object in the second assignment, it modifies the original object. Then you could do:
console.log(object.object.object.object === object); // true
For an array, it would be:
array = [];
array[0] = array;
console.log(array[0][0][0][0][0] === array); // true
There are two assignments to window.object. The first one creates a new object and assigns it to window.object. That one has a property called "object" whose value is undefined, because at the time the object literal is evaluated window.object is undefined.
The second assignment instantiates a new object. That one also has a property called "object", whose value is the object created in the first assignment.
Thus, window.object is not the same object as window.object.object.
The key is that in an assignment like this:
object = { object: window.object };
The value of the "object" property in the object literal is evaluated before the assignment takes place.
Let explain it line by line
object = {object : window.object};
//here object.object is undefined and object new object so object !== object.object
//lets obj1 = {object: undefined}
object = {object : window.object};
//here object is new instance of Object while object.object is equal to older object
//basically object != object.object
// obj2 = {object: obj1}
console.log(object === object); // true because same object obj2 == obj2
console.log(object.object === object); // false because two different object as obj1 != obj2
console.log(object.object === object.object); // true because same object obj1 == obj1
console.log(object.object.object === object.object); // false because undefined != obj1
I'm trying to detect is my variable is object.
I have that code:
var obj = {
'propA': 'a',
'propB': {
'underB': 'underBB',
'underB2': 'underBB2'
}
};
now when I checking: obj === Object I'm getting false even if when I type obj in console I see it's object:
Object {propA: "a", propB: Object}
DEMO
Questions:
How I can check if obj is a Object?
Why when I try compare my var and Object I'm getting false?
You can use:
if(typeof obj == 'object')
See here
This will also return true if obj is an array so sometimes its good to use instanceof to make sure its an object of the 'type' you want.
you can use instance of to determine type of an object.try this :
console.log(obj instanceof Object);
Besides, you are comparing object instance with a contractor(function) with === operator.this will always be false.