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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 2017 Draft (ECMA-262)
The definition of 'null value' in that specification.
Draft  

瀏覽器相容性

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)

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