null
The value null represents the intentional absence of any object value. It is one of JavaScript's primitive values (en-US).
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 (ECMA-262) The definition of 'null value' in that specification. |
Living Standard |
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! (en-US)
| 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) |