值 null
特指对象的值未设置。它是 JavaScript 基本类型 之一。
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语法
null
描述
值 null
是一个字面量,不像 undefined
,它不是全局对象的一个属性。null
是表示缺少的标识,指示变量未指向任何对象。把 null
作为尚未创建的对象,也许更好理解。在 API 中,null
常在返回类型应是一个对象,但没有关联的值的地方使用。
// foo 不存在,它从来没有被定义过或者是初始化过: foo; "ReferenceError: foo is not defined" // foo 现在已经是知存在的,但是它没有类型或者是值: var foo = null; foo; null
null
与 undefined
的不同点:
当检测 null
或 undefined
时,注意相等(==)与全等(===)两个操作符的区别 ,前者会执行类型转换:
typeof null // "object" (因为一些以前的原因而不是'null') typeof undefined // "undefined" null === undefined // false null == undefined // true null === null // true null == null // true !null //true isNaN(1 + null) // false isNaN(1 + undefined) // true
规范
规范版本 | 规范状态 | 注解 |
---|---|---|
ECMAScript 1st Edition (ECMA-262) | Standard | Initial definition. |
ECMAScript 5.1 (ECMA-262) null value |
Standard | |
ECMAScript 2015 (6th Edition, ECMA-262) null value |
Standard | |
ECMAScript Latest Draft (ECMA-262) null value |
Draft |
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.
Update compatibility data on GitHub
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
null |
Chrome Full support 1 | Edge Full support 12 | Firefox Full support 1 | IE Full support 3 | Opera Full support Yes | Safari Full support Yes | WebView Android Full support 1 | Chrome Android Full support 18 | Firefox Android Full support 4 | Opera Android Full support Yes | Safari iOS Full support Yes | Samsung Internet Android Full support 1.0 | nodejs Full support Yes |
Legend
- Full support
- Full support