The valueOf()
method returns the primitive value of the specified object.
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Syntax
object.valueOf()
Return value
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.
Description
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:
myNumberType.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.
Examples
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
Specifications
Specification |
---|
ECMAScript (ECMA-262) The definition of 'Object.prototype.valueOf' in that specification. |
Browser compatibility
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
valueOf | Chrome Full support 1 | Edge Full support 12 | Firefox Full support 1 | IE Full support 4 | Opera Full support 3 | Safari Full support 1 | WebView Android Full support 1 | Chrome Android Full support 18 | Firefox Android Full support 4 | Opera Android Full support 10.1 | Safari iOS Full support 1 | Samsung Internet Android Full support 1.0 | nodejs Full support Yes |
Legend
- Full support
- Full support