JS: Object .prototype .valueOf

Object.prototype.valueOf

Object.prototype.valueOf

return the value of this (binding) it receives. e.g. myObj.valueOf() === myObj

If type of this (binding) is not an object type, either throw an error, or convert it to an object type, return it.

It has similar behavior as the Object Constructor Object(arg), except in the Object(arg) case, if arg is null or undefined, it creates a empty object, while in the valueOf() case, it throws TypeError.

It's usually called in one of the following form:

• Object.prototype.valueOf.call(x)
• Reflect.apply(Object.prototype.valueOf, x, [])

[see Function Call, Apply, Bind]

Here is a table showing the conversion:

Value	Result
undefined	throw TypeError
null	throw TypeError
true or false	boolean object
number	number object
string	string object
symbol	symbol object
object	object

const jj = {};
console.assert(jj.valueOf() === jj);

Edge case. on primitive

// testing use of Object.prototype.valueOf but with a primitive value as argument

console.assert(
 typeof Reflect.apply(
  Object.prototype.valueOf,
  true,
  [],
 ) === "object",
);

console.assert(
 typeof Reflect.apply(
  Object.prototype.valueOf,
  3,
  [],
 ) === "object",
);

console.assert(
 typeof Reflect.apply(
  Object.prototype.valueOf,
  "abc",
  [],
 ) === "object",
);

Edge cases. on null

// Object.prototype.valueOf on null

try {
 Reflect.apply(Object.prototype.valueOf, null, []);
} catch (xerror) {
 console.log(xerror);
}

// TypeError: Cannot convert undefined or null to object