Professional JavaScript for Web Developers, 4th Edition

4Variables, Scope, and Memory

WHAT'S IN THIS CHAPTER?

• Working with primitive and reference values in variables
• Understanding execution context
• Understanding garbage collection

WROX.COM DOWNLOADS FOR THIS CHAPTER

Please note that all the code examples for this chapter are available as a part of this chapter's code download on the book's website at www.wrox.com/go/projavascript4e on the Download Code tab.

The nature of variables in JavaScript, as defined in ECMA-262, is quite unique compared to that of 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 atomic pieces of data, while reference values are objects that may be made up of multiple values.

When a value is assigned to a variable, the JavaScript engine must determine if it's a primitive or a reference value. The six primitive types were discussed in the previous chapter: Undefined, Null, Boolean, Number, String, and Symbol. These variables are said to be accessed by value, because you are manipulating the actual value ...