Which way should be used for null comparison in JavaScript

Question

To check whether an object is null or not in javascript, i have seen two ways,

let obj = {};

Option 1

if(obj){ console.log('object exists'); }

Option 2

if(!!obj === true){ console.log('object exists'); }

Is there any advantage of choosing one option over the other? which option is preferred as the standard code ?

Possibly duplicate of https://stackoverflow.com/questions/6003884/how-do-i-check-for-null-values-in-javascript — Jasmin Mistry, Aug 14 '18 at 04:30
You are not checking null. You are checking if obj has a truthy value. — Jagdeep Singh, Aug 14 '18 at 04:33
Possible duplicate of [How do I check for null values in JavaScript?](https://stackoverflow.com/questions/6003884/how-do-i-check-for-null-values-in-javascript) — 31piy, Aug 14 '18 at 04:35
@31piy `typeof null === "object"`, so you might not want to use it, unless you are checking if `obj` is either undeclared or actually `undefined`. — VLAZ, Aug 14 '18 at 04:57

score 3 · Accepted Answer · answered Aug 14 '18 at 04:40

Use simple ===.

In your code, you are not checking if the object is null or not. Instead, your code just checks if it's not a Falsy value. Note that there are a total of 6 Falsy Values in JavaScript, null being one of those. Read more about them here. If you just want to check if a variable is null, the following is the best way to go:

if (obj === null) { 
  console.log("object is null");
} else {
  console.log("object is not null");
}

score 2 · Answer 2 · answered Aug 14 '18 at 04:38

2

If you only want to check if the object is null:

if(obj === null) {
...
}

answered Aug 14 '18 at 04:38

jh314

27,144
16
62
82

Parama Kar · Answer 3 · 2018-08-14T04:58:40.220

1

An object in javascript is always truthy irrespective of whether it has any property defined on it.

Thus,

var obj={}
if(obj){
//this block will always be executed.
}

If you want to check if an object has been defined in the current lexical scope try:

if(typeof(obj) !== 'undefined' && obj !== null){ 
  console.log('object exists'); 
}
else{
  console.log('nope')
}

If you want to check if an object has any property on it or it is an empty object try:

if(typeof(obj) !== 'undefined' && obj !== null){ 
  console.log('object exists'); 
}
else{
  if(Object.keys(obj).length){
    //do something
  }
}

edited Aug 14 '18 at 04:58

answered Aug 14 '18 at 04:48

Parama Kar

462
3
8

`typeof` always returns a `String` so `typeof(obj) === undefined` will **always** return `false`. – connexo Aug 14 '18 at 04:51
`typeof null` will return `"object"`, so `null` will pass your test, too. – connexo Aug 14 '18 at 04:54
`Object.keys(obj)` will throw an error if `obj` is `undefined` or `null`. – connexo Aug 14 '18 at 04:58

traktor · Answer 4 · 2018-08-14T07:11:33.190

To test if a value obj is of Object data type, try

if( obj and typeof obj === "object") {
     // obj is an object and it's safe to access properties
}

The truthiness of obj indicates it's not null, and typeof returning "object" means it's not some other primitive data type.

Note that null is its own data type in JavaScript (null), but typeof null returns "object" because of limitations in the early implementations of the language.

Conversely, to test if a value is null and not an object, simply use a strict equality test:

if( obj === null) {
   // the data type of obj is null
   ...
}

score -3 · Answer 5 · answered Aug 14 '18 at 04:40

-3

Choosing a comparison method varies on how you predict how the data type will be, but if you want a really safe standard, use 'typeof'. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/typeof

if( typeof obj === "object" ){ console.log('object exists'); }

if( typeof obj !== "undefined" ){ console.log('object exists'); }

answered Aug 14 '18 at 04:40

CodeYute

64
4

3

But...that fails precisely for when an object is `null`! Because `typeof null === 'object'` – VLAZ Aug 14 '18 at 04:43

Which way should be used for null comparison in JavaScript

5 Answers5