Object.prototype.valueOf()

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概览

The valueOf() method returns the primitive value of the specified object.

语法

object.valueOf()

描述

JavaScript calls the valueOf method to convert an object to a primitive value. You rarely need to invoke the valueOf method yourself; JavaScript automatically invokes it when encountering an object where a primitive value is expected.

By default, the valueOf method is inherited by every object descended from Object. Every built-in core object overrides this method to return an appropriate value. If an object has no primitive value, valueOf returns the object itself, which is displayed as:

[object Object]
 

You can use valueOf within your own code to convert a built-in object into a primitive value. When you create a custom object, you can override Object.prototype.valueOf() to call a custom method instead of the default Object method.

Overriding valueOf for custom objects

You can create a function to be called in place of the default valueOf method. Your function must take no arguments.

Suppose you have an object type myNumberType and you want to create a valueOf method for it. The following code assigns a user-defined function to the object's valueOf method:

myNumberType.prototype.valueOf = function() { return customPrimitiveValue; };
 

With the preceding code in place, any time an object of type myNumberType is used in a context where it is to be represented as a primitive value, JavaScript automatically calls the function defined in the preceding code.

An object's valueOf method is usually invoked by JavaScript, but you can invoke it yourself as follows:

myNumber.valueOf()
 

Note: Objects in string contexts convert via the toString() method, which is different from String objects converting to string primitives using valueOf. All objects have a string conversion, if only "[object type]". But many objects do not convert to number, boolean, or function.

示例

Example: Using valueOf

o = new Object();
myVar = o.valueOf();      // [object Object]
 
 

规范

Specification Status Comment
ECMAScript 1st Edition. Standard Initial definition. Implemented in JavaScript 1.1.
ECMAScript 5.1 (ECMA-262)
Object.prototype.valueOf
Standard  
ECMAScript 6 (ECMA-262)
Object.prototype.valueOf
Release Candidate  

浏览器兼容性

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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