When is null or undefined used in JavaScript?
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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 .
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:
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
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
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