The valueOf method returns a primitive value for a specified object and is inherited by all objects descended from Object. It is usually called automatically by JavaScript behind the scenes whenever it encounters an object where a primitive value is expected. If the object has no primitive value, then the object itself is returned as [object Object]. You can also call valueOf yourself to convert a built-in object into a primitive value. The following two examples illustrate uses of it:

Code:
(Object.valueOf()

Output:
function Object() { [native code] }

Code:
function Cat(breed, name, age)
{
   this.breed = breed
   this.name = name
}
Cat.valueOf()

Output:
function Cat(breed, name, age) { this.breed = breed this.name = name }

The valueOf method can also be overwritten in a custom object by assigning a user-defined function with no arguments in its place as follows:

Code:
Cat.prototype.valueOf() = myValueOf()

NOTE:

Every core JavaScript object will over-ride the valueOf method to return an appropriate value.