value_of.md

How to know the valueOf() method Back

In ECMAScript 5, the method valueOf() is defined as followed:

when the valueOf method is called, the following steps are taken:

1. Let O be the result of calling ToObject passing the this value as the argument.

2. If O is the result of calling the Object constructor with a host object, then return either O or another value such as the host object originally passed to the constructor. The specific result that is returned is implementation-defined.

3. return O.

To put it simple, this method will call ToObject(this), and return different things according to this:

• this is the original object, returning will depends on different browsers.
• this is not the original object, returning will return the mothod ToObject(this).

Parameter Types	Return
undefined	Throw TypeError Exception
null	Throw TypeError Exception
Number	Create a Number Object and pass this into the constructor
String	Create a String Object and pass this into the constructor
Boolean	Create a Boolean Object and pass this into the constructor
Object	this

Object Types	Return
undefined	Throw TypeError Exception
null	Throw TypeError Exception
Number	Create a Number Object whose value is equivalent to the object itself
String	Create a String Object whose value is equivalent to the object itself
Boolean	Create a Boolean Object whose value is equivalent to the object itself
Object	The Object itself

Example

var num = 123;
console.log(num.valueOf());                    /** => 123           */
console.log(typeof num);                       /** => number        */
console.log(typeof num.valueOf());             /** => number        */

var num = new Number(123);
console.log(num.valueOf());                    /** => 123           */
console.log(typeof num);                       /** => object        */
console.log(typeof num.valueOf());             /** => number        */