由於本文件沒有此語言的翻譯版本,您閱讀的是英文版的內容。 幫助我們翻譯這篇文章!
The value null
represents the intentional absence of any object value. It is one of JavaScript's primitive values.
語法
null
描述
The value null
is written with a literal, null
(it's not an identifer for a property of the global object like undefined
can be). In APIs, null
is often retrieved in place where an object can be expected but no object is relevant. When checking for null or undefined beware of the differences between equality (==) and identity (===) operators (type-conversion is performed with the former).
// foo does not exist. It is not defined and has never been initialized: > foo "ReferenceError: foo is not defined" // foo is known to exist now but it has no type or value: > var foo = null; foo "null"
Difference between null
and undefined
typeof null // object (bug in ECMAScript, should be null) typeof undefined // undefined null === undefined // false null == undefined // true
規範
Specification | Status | Comment |
---|---|---|
ECMAScript 1st Edition (ECMA-262) | Standard | Initial definition. |
ECMAScript 5.1 (ECMA-262) The definition of 'null value' in that specification. |
Standard | |
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'null value' in that specification. |
Standard | |
ECMAScript Latest Draft (ECMA-262) The definition of 'null value' in that specification. |
Draft |
瀏覽器相容性
We're converting our compatibility data into a machine-readable JSON format.
This compatibility table still uses the old format,
because we haven't yet converted the data it contains.
Find out how you can help!
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) |