undefined | JavaScript

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描述
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规范
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全局 undefined 特性表示原语值 undefined 。它是某一 JavaScript 原语类型 .

特性属性在 `undefined`
可写	no
可枚举	no
可配置	no

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句法

undefined

描述

undefined 是特性在 全局对象 . 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 .

小心。 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');

范例

严格相等和 undefined

可以使用 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
}

注意： 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 .

见 comparison operators 了解细节。

typeof 运算符和 undefined

Alternatively, typeof 可以被使用：

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 全局对象 , so checking the existence of a variable in the global context can be done by checking the existence of a property on the 全局对象 ，使用 in operator, for instance:

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

void 运算符和 undefined

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
}

规范

规范
ECMAScript (ECMA-262) 在该规范中的 undefined 定义。

浏览器兼容性

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. 更新 GitHub 上的兼容性数据

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`undefined`	Chrome 1	Edge 12	Firefox 1	IE 5.5	Opera 3	Safari 1	WebView Android 1	Chrome Android 18	Firefox Android 4	Opera Android 10.1	Safari iOS 1	Samsung Internet Android 1.0	nodejs 0.1.100

图例

完整支持

另请参阅

JavaScript 的原语类型
null

元数据

最后修改： Aug 3, 2020

在此页

句法

描述

范例

严格相等和 undefined

typeof 运算符和 undefined

void 运算符和 undefined

规范

浏览器兼容性

图例

另请参阅

相关话题