Object.prototype.valueOf() - JavaScript

Try it

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

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

const object1 = new MyNumberType(4);

console.log(object1 + 3);
/ Expected output: 7

Syntax

valueOf()

Return value

The this value, converted to an object.

Note: In order for valueOf to be useful during type conversion, it must return a primitive. Because all primitive types have their own valueOf() methods, calling aPrimitiveValue.valueOf() generally does not invoke Object.prototype.valueOf().

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.

This method is called in priority by Object.prototype.toString() base implementation), so valueOf() is usually not called in this case.

All objects that inherit from Object.prototype (that is, all except [Symbol.toPrimitive]() method, which allows even more control over the conversion process, and will always be preferred over valueOf or toString for any type conversion.

Examples

Using valueOf()

The base valueOf() method returns the this value itself, converted to an object if it isn't already. Therefore its return value will never be used by any primitive conversion algorithm.

const obj = { foo: 1 };
console.log(obj.valueOf() === obj); / true

console.log(Object.prototype.valueOf.call("primitive"));
/ [String: 'primitive'] (a wrapper object)

Overriding valueOf for custom objects

You can create a function to be called in place of the default valueOf method. Your function should take no arguments, since it won't be passed any when called during type conversion.

For example, you can add a valueOf method to your custom class Box.

class Box {
  #value;
  constructor(value) {
    this.#value = value;
  }
  valueOf() {
    return this.#value;
  }
}

With the preceding code in place, any time an object of type Box is used in a context where it is to be represented as a primitive value (but not specifically a string), JavaScript automatically calls the function defined in the preceding code.

const box = new Box(123);
console.log(box + 456); / 579
console.log(box == 123); / true

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

box.valueOf();

Using unary plus on objects

[Symbol.toPrimitive](), means calling its valueOf(). However, if the object doesn't have a custom valueOf() method, the base implementation will cause valueOf() to be ignored and the return value of toString() to be used instead.

+new Date(); / the current timestamp; same as new Date().getTime()
+{}; / NaN (toString() returns "[object Object]")
+[]; / 0 (toString() returns an empty string list)
+[1]; / 1 (toString() returns "1")
+[1, 2]; / NaN (toString() returns "1,2")
+new Set([1]); / NaN (toString() returns "[object Set]")
+{ valueOf: () => 42 }; / 42

Specifications

Specification
ECMAScript® 2026 Language Specification # sec-object.prototype.valueof