Object.prototype.valueOf()

Try it

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

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

const object = new MyNumberType(4);

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

Syntax

valueOf()

Parameters

None.

Return value

The this value, converted to an object.

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 numeric conversion and primitive conversion, but string conversion calls toString() in priority, and toString() is very likely to return a string value (even for the 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 null-prototype objects) inherit the toString() method. The Object.prototype.valueOf() base implementation is deliberately useless: by returning an object, its return value will never be used by any primitive conversion algorithm. Many built-in objects override this method to return an appropriate primitive value. When you create a custom object, you can override valueOf() to call a custom method, so that your custom object can be converted to a primitive value. Generally, valueOf() is used to return a value that is most meaningful for the object — unlike toString(), it does not need to be a string. Alternatively, you can add a [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

Unary plus performs number coercion on its operand, which, for most objects without [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

Browser compatibility

See also

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