Object.prototype.valueOf()
The valueOf()
method returns the primitive value of the
specified object.
Syntax
valueOf()
Return value
The primitive value of the specified object.
Note: 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 Language Specification # sec-object.prototype.valueof |
Browser compatibility
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