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• 句法
• 描述
• 范例
• 规范
• 浏览器兼容性
• 另请参阅
• 相关话题

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

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

句法

object.valueOf()
					

返回值

The primitive value of the specified object.

A (unary) plus sign can sometimes be used as a shorthand for valueOf , e.g. in +new Number() ). Also see Using unary plus .

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.

默认情况下， 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.

可以使用 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 方法。

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:

myNumberType.valueOf()
					

注意： 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.

范例

Using valueOf on custom types

function MyNumberType(n) {
    this.number = n;
}
MyNumberType.prototype.valueOf = function() {
    return this.number;
};
var myObj = new MyNumberType(4);
myObj + 3; // 7
					

Using unary plus

+"5" // 5 (string to number)
+"" // 0 (string to number)
+"1 + 2" // NaN (doesn't evaluate)
+new Date() // same as (new Date()).getTime()
+"foo" // NaN (string to number)
+{} // NaN
+[] // 0 (toString() returns an empty string list)
+[1] // 1
+[1,2] // NaN
+new Set([1]) // NaN
+BigInt(1) // Uncaught TypeError: Cannot convert a BigInt value to a number
+undefined // NaN
+null // 0
+true // 1
+false // 0
					

规范

规范
ECMAScript (ECMA-262) The definition of 'Object.prototype.valueOf' in that specification.

浏览器兼容性

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. 更新 GitHub 上的兼容性数据

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
valueOf	Chrome 1	Edge 12	Firefox 1	IE 4	Opera 3	Safari 1	WebView Android 1	Chrome Android 18	Firefox Android 4	Opera Android 10.1	Safari iOS 1	Samsung Internet Android 1.0	nodejs Yes

图例

完整支持

完整支持

另请参阅

• Object.prototype.toString()
• parseInt()
• Symbol.toPrimitive

元数据

• 最后修改： Jun 8, 2020

相关话题

1. 标准内置对象
2. Object
3. 特性
   1. Object.prototype.__proto__
   2. Object.prototype.constructor
4. 方法
   1. Object.assign()
   2. Object.create()
   3. Object.defineProperties()
   4. Object.defineProperty()
   5. Object.entries()
   6. Object.freeze()
   7. Object.fromEntries()
   8. Object.getOwnPropertyDescriptor()
   9. Object.getOwnPropertyDescriptors()
   10. Object.getOwnPropertyNames()
   11. Object.getOwnPropertySymbols()
   12. Object.getPrototypeOf()
   13. Object.is()
   14. Object.isExtensible()
   15. Object.isFrozen()
   16. Object.isSealed()
   17. Object.keys()
   18. Object.preventExtensions()
   19. Object.prototype.__defineGetter__()
   20. Object.prototype.__defineSetter__()
   21. Object.prototype.__lookupGetter__()
   22. Object.prototype.__lookupSetter__()
   23. Object.prototype.hasOwnProperty()
   24. Object.prototype.isPrototypeOf()
   25. Object.prototype.propertyIsEnumerable()
   26. Object.prototype.toLocaleString()
   27. Object.prototype.toSource()
   28. Object.prototype.toString()
   29. Object.prototype.valueOf()
   30. Object.seal()
   31. Object.setPrototypeOf()
   32. Object.values()
5. 继承：
6. Function
7. 特性
   1. Function.arguments
   2. Function.caller
   3. Function.displayName
   4. Function.length
   5. Function.name
8. 方法
   1. Function.prototype.apply()
   2. Function.prototype.bind()
   3. Function.prototype.call()
   4. Function.prototype.toSource()
   5. Function.prototype.toString()