undefined

Summary

The global undefined property represents the value undefined.

Property attributes of undefined
Writable No
Enumerable No
Configurable No

Syntax

undefined

Description

undefined is a property of the global object, i.e. it is a variable in global scope. The initial value of undefined is the primitive value undefined.

In modern browsers (JavaScript 1.8.5 / Firefox 4+), undefined is a non-configurable, non-writable property per the ECMAScript 5 specification. Even when this is not the case, avoid overriding it.

A variable that has not been assigned a value is of type undefined. A method or statement also returns undefined if the variable that is being evaluated does not have an assigned value. A function returns undefined if a value was not returned.

Since undefined is not a reserved word, it can be used as an identifier (variable name) in any scope other than the global scope.

// logs "foo string"
(function(){ var undefined = 'foo'; console.log(undefined, typeof undefined); })();

// logs "foo string"
(function(undefined){ console.log(undefined, typeof undefined); })('foo');

Examples

Strict equality and undefined

You can use undefined and the strict equality and inequality operators to determine whether a variable has a value. In the following code, the variable x is not defined, and the if statement evaluates to true.

var x;
if (x === undefined) {
   // these statements execute
}
else {
   // these statements do not execute
}
Note: The strict equality operator rather than the standard equality operator must be used here, because x == undefined also checks whether x is null, while strict equality doesn't. null is not equivalent to undefined. See comparison operators for details.

Typeof operator and undefined

Alternatively, typeof can be used:

var x;
if (typeof x === 'undefined') {
   // these statements execute
}

One reason to use typeof is that it does not throw an error if the variable has not been defined.

// x has not been defined before
if (typeof x === 'undefined') { // evaluates to true without errors
   // these statements execute
}

if(x === undefined){ // throws a ReferenceError

}

However, this kind of technique should be avoided. JavaScript is a statically scoped language, so knowing if a variable is defined can be read by seeing whether it is defined in an enclosing context. The only exception is the global scope, but the global scope is bound to the global object, so checking the existence of a variable in the global context can be done by checking the existence of a property on the global object (using the in operator, for instance).

Void operator and undefined

The void operator is a third alternative.

var x;
if (x === void 0) {
   // these statements execute
}

// y has not been defined before
if (y === void 0) {
   // throws a ReferenceError (in contrast to `typeof`)
}

Specifications

Specification Status Comment
ECMAScript 1st Edition. Standard Initial definition. Implemented in JavaScript 1.3
ECMAScript Language Specification 5.1th Edition (ECMA-262) Standard  
ECMAScript Language Specification 6th Edition (ECMA-262) Draft  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support (Yes) (Yes) (Yes) (Yes) (Yes)
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)

 

Document Tags and Contributors

Contributors to this page: Sevenspade
Last updated by: Sevenspade,