Table of contents
- 1. Summary
- 2. Syntax
- 3. Description
Summary
The value undefined.
| Core Global Property | |
|---|---|
| Implemented in | JavaScript 1.3 |
| ECMAScript Edition | ECMAScript 1st Edition |
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.
JavaScript 1.8.5 note
Starting in JavaScript 1.8.5 (Firefox 4), undefined is non-writable, as per the ECMAScript 5 specification.
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.
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
}
if (x !== undefined) {
// these statements do not execute
}
x == undefined also checks whether x is null, while strict equality doesn't. null is not equivalent to undefined. See comparison operators for details.Alternatively, typeof can be used:
var x;
if (typeof x == 'undefined') {
// these statements execute
}
