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



Return value

The primitive value of the specified object.


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.

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:


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.


Using valueOf

function MyNumberType(n) {
    this.number = n;

MyNumberType.prototype.valueOf = function() {
    return this.number;

var myObj = new MyNumberType(4);
myObj + 3; // 7


Specification Status Comment
ECMAScript 1st Edition (ECMA-262) Standard Initial definition. Implemented in JavaScript 1.1.
ECMAScript 5.1 (ECMA-262)
The definition of 'Object.prototype.valueOf' in that specification.
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Object.prototype.valueOf' in that specification.
ECMAScript Latest Draft (ECMA-262)
The definition of 'Object.prototype.valueOf' in that specification.
Living Standard  

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic Support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)
FeatureAndroidChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic Support (Yes) (Yes) (Yes) (Yes) (Yes) (Yes) (Yes)

See also

Tag del documento e collaboratori

 Ultima modifica di: fscholz,