JavaScript: The Definitive Guide, 6th Edition

Wrapper Objects

JavaScript objects are composite values: they are a collection of properties or named values. We refer to the value of a property using the . notation. When the value of a property is a function, we call it a method. To invoke the method m of an object o, we write o.m().

We’ve also seen that strings have properties and methods:

var s = "hello world!";                             // A string
var word = s.substring(s.indexOf(" ")+1, s.length); // Use string properties

Primitive strings are not objects, though, so why do they have properties? Whenever you try to refer to a property of a string s, JavaScript converts the string value to an object as if by calling new String(s). This object inherits (see Inheritance) string methods and is used to resolve the property reference. Once the property has been resolved, the newly created object is discarded. (Implementations are not required to actually create and discard this transient object: they must behave as if they do, however.)

Numbers and booleans have methods for the same reason that strings do: a temporary object is created using the Number() or Boolean() constructor, and the method is resolved using that temporary object. There are no wrapper objects for the null and undefined values: any attempt to access a property of one of these values causes a TypeError.

Consider the following code and think about what happens when it is executed:

var s = "test";         // Start with a string value.
s.len = 4;              // Set a property on it.
var t = s.len;          // Now query the ...