undefined - JavaScript 编辑

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

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

undefined

Description

undefined is a property of the global object. That is, 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.

Be careful. 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 initialized, 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 (as opposed to the standard equality operator) must be used here, because x == undefined also checks whether x is null, while strict equality doesn't. This is because 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, there is another alternative. 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 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:

if ('x' in window) {
  //  these statements execute only if x is defined globally
}

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 Uncaught ReferenceError: y is not defined
}

Specifications

Specification
ECMAScript (ECMA-262) The definition of 'undefined' in that specification.

Browser compatibility

BCD tables only load in the browser

See also

• JavaScript's primitive types
• null