When is null or undefined used in JavaScript?

I am really confused as to when JavaScript returns null or undefined. Also different browsers seem to be returning these differently.

Could you please give some examples of null/undefined with the browsers that return them.

While I am now clear on the undefined aspect, I am still not 100% clear on null. Is it similar to a blank value?

E.g. You have a text box which does not have any value set. Now when you try to access its value, will it be null or undefined and are they similar?

I find that some of these answers are vague and complicated, I find the best way to figure out these things for sure is to just open up the console and test it yourself.

var x;

x == null            // true
x == undefined       // true
x === null           // false
x === undefined      // true

var y = null;
 
y == null            // true
y == undefined       // true
y === null           // true
y === undefined      // false

typeof x             // 'undefined'
typeof y             // 'object'

var z = {abc: null};

z.abc == null        // true
z.abc == undefined   // true
z.abc === null       // true
z.abc === undefined  // false

z.xyz == null        // true
z.xyz == undefined   // true
z.xyz === null       // false
z.xyz === undefined  // true

null = 1;            // throws error: invalid left hand assignment
undefined = 1;       // works fine: this can cause some problems

So this is definitely one of the more subtle nuances of JavaScript. As you can see, you can override the value of undefined, making it somewhat unreliable compared to null. Using the == operator, you can reliably use null and undefined interchangeably as far as I can tell. However, because of the advantage that null cannot be redefined, I might would use it when using ==.

For example, variable != null will ALWAYS return false if variable is equal to either null or undefined, whereas variable != undefined will return false if variable is equal to either null or undefined UNLESS undefined is reassigned beforehand.

You can reliably use the === operator to differentiate between undefined and null, if you need to make sure that a value is actually undefined (rather than null).

According to the ECMAScript 5 spec:

• Both Null and Undefined are two of the six built in types.

4.3.9 undefined value

>primitive value used when a variable has not been assigned a value

4.3.11 null value

>primitive value that represents the intentional absence of any object value

The DOM methods getElementById(), nextSibling(), childNodes[n], parentNode() and so on return null (defined but having no value) when the call does not return a node object.

The property is defined, but the object it refers to does not exist.

This is one of the few times you may not want to test for equality-

if(x!==undefined) will be true for a null value

but if(x!= undefined) will be true (only) for values that are not either undefined or null.

You get undefined for the various scenarios:

You declare a variable with var but never set it.

var foo; 
alert(foo); //undefined.

You attempt to access a property on an object you've never set.

var foo = {};
alert(foo.bar); //undefined

You attempt to access an argument that was never provided.

function myFunction (foo) {
  alert(foo); //undefined.
}

As cwolves pointed out in a comment on another answer, functions that don't return a value.

function myFunction () {
}
alert(myFunction());//undefined

A null usually has to be intentionally set on a variable or property (see comments for a case in which it can appear without having been set). In addition a null is of type object and undefined is of type undefined.

I should also note that null is valid in JSON but undefined is not:

JSON.parse(undefined); //syntax error
JSON.parse(null); //null

I might be missing something, but afaik, you get undefined only

Update: Ok, I missed a lot, trying to complete:

You get undefined...

... when you try to access properties of an object that don't exist:

var a = {}
a.foo // undefined

... when you have declared a variable but not initialized it:

var a;
// a is undefined

... when you access a parameter for which no value was passed:

function foo (a, b) {
    // something
}

foo(42); // b inside foo is undefined

... when a function does not return a value:

function foo() {};
var a = foo(); // a is undefined

---

It might be that some built-in functions return null on some error, but if so, then it is documented. null is a concrete value in JavaScript, undefined is not.

---

Normally you don't need to distinguish between those. Depending on the possible values of a variable, it is sufficient to use if(variable) to test whether a value is set or not (both, null and undefined evaluate to false).

> Also different browsers seem to be returning these differently.

Please give a concrete example.

Regarding this topic the specification (ecma-262) is quite clear

I found it really useful and straightforward, so that I share it:

• Here you will find Equality algorithm
• Here you will find Strict equality algorithm

I bumped into it reading "Abstract equality, strict equality, and same value" from mozilla developer site, section sameness.

I hope you find it useful.

A property, when it has no definition, is undefined. null is an object. It's type is null. undefined is not an object, its type is undefined.

This is a good article explaining the difference and also giving some examples.

null vs undefined