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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




    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');


    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`)


    Specification Status Comment
    ECMAScript 1st Edition. Standard Initial definition. Implemented in JavaScript 1.3
    ECMAScript 5.1 (ECMA-262)
    The definition of 'undefined' in that specification.
    ECMAScript 6 (ECMA-262)
    The definition of 'undefined' in that specification.
    Release Candidate  

    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

    Last updated by: Korikulum,
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