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The global undefined
property represents the primitive value undefined
. It is one of JavaScript's primitive types.
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
.
While it is possible to use it as an identifier (variable name) in any scope other than the global scope (because undefined
is not a reserved word
), doing so is a very bad idea that will make your code difficult to maintain and debug.
//DON'T DO THIS // 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 declared.
// x has not been declared 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 declared can be read by seeing whether it is declared 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 declared before if (y === void 0) { // throws a ReferenceError (in contrast to `typeof`) }
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript 1st Edition (ECMA-262) The definition of 'undefined' in that specification. |
Standard | Initial definition. Implemented in JavaScript 1.3. |
ECMAScript 5.1 (ECMA-262) The definition of 'undefined' in that specification. |
Standard | |
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'undefined' in that specification. |
Standard | |
ECMAScript 2017 Draft (ECMA-262) The definition of 'undefined' in that specification. |
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) |