null - JavaScript 编辑

The value null represents the intentional absence of any object value. It is one of JavaScript's primitive values and is treated as falsy for boolean operations.

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Syntax

null

Description

The value null is written with a literal: null. null is not an identifier for a property of the global object, like undefined can be. Instead, null expresses a lack of identification, indicating that a variable points to no object. In APIs, null is often retrieved in a place where an object can be expected but no object is relevant. 

// 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

Examples

Difference between null and undefined

When checking for null or undefined, beware of the differences between equality (==) and identity (===) operators, as the former performs type-conversion.

typeof null          // "object" (not "null" for legacy reasons)
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

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• undefined
• NaN

• void operator