Professional, JavaScript® for Web Developers, Second Edition

Chapter 4. Variables, Scope, and Memory

The nature of variables in JavaScript, as defined in ECMA-262 Third Edition, is quite unique compared to other languages. Being loosely typed, a variable is literally just a name for a particular value at a particular time. Because there are no rules defining the type of data that a variable must hold, a variable's value and data type can change during the lifetime of a script. Though this is an interesting, powerful, and problematic feature, there are many more complexities related to variables.

Primitive and Reference Values

ECMAScript variables may contain two different types of data: primitive values and reference values. Primitive values are simple pieces of data that are stored in memory on the stack, which is to say that the value is completely stored in one memory location. Reference values, on the other hand, are objects that are stored on the heap, meaning that the value stored in the variable is actually just a pointer to another memory location where the object is stored.

When a value is assigned to a variable, the interpreter must determine if it's a primitive or a reference. The five primitive types were discussed in the previous chapter: Undefined, Null, Boolean, Number, and String. Each of these data types takes up a fixed amount of space, so values can easily be stored on the stack. Doing so also allows for fast variable lookup. These variables are said to be accessed by value, because you are manipulating the actual value stored ...