JavaScript: The Definitive Guide, Fourth Edition

Name

Object.valueOf( ) — the primitive value of the specified object

Availability

JavaScript 1.1; JScript 2.0; ECMAScript v1

Synopsis

                  object.valueOf( )

Returns

The primitive value associated with the object, if any. If there is no value associated with object, returns the object itself.

Description

The valueOf( ) method of an object returns the primitive value associated with that object, if there is one. For objects of type Object there is no primitive value, and this method simply returns the object itself.

For objects of type Number, however, valueOf( ) returns the primitive numeric value represented by the object. Similarly, it returns the primitive boolean value associated with a Boolean object and the string associated with a String object.

It is rarely necessary to invoke the valueOf( ) method yourself. JavaScript does this automatically whenever an object is used where a primitive value is expected. In fact, because of this automatic invocation of the valueOf( ) method, it is difficult to even distinguish between primitive values and their corresponding objects. The typeof operator shows you the difference between strings and String objects for example, but in practical terms, you can use them equivalently in your JavaScript code.

The valueOf( ) methods of the Number, Boolean, and String objects convert these wrapper objects to the primitive values they represent. The Object( ) constructor performs the opposite operation when invoked with a number, boolean, or string argument: ...