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The JavaScript Language 481

17

Let’s break down a variable declaration into pieces. Here’s a declaration:

var message = "My message";

The line begins with var, which indicates that this is a variable declaration. The name
of this variable is message. There are a number of rules that apply to naming variables,
which I will list shortly. The assignment operator (=) is used to assign a value to the
variable when it’s declared. The value returned by the expression on the right side of the
operator is assigned to the newly declared variable.

Variable names must conform to the following rules:

n Variable names can include only letters, numbers, and the underscore (_) or dollar

sign ($) character.

n Variable names cannot start with a number.

n You cannot use reserved words as a variable name. Reserved words are words that

have a specific meaning for the JavaScript interpreter. For example, naming a vari-

able var won’t work. Table 17.2 contains a full list of JavaScript reserved words.

n As a matter of style, JavaScript variables begin with a lowercase letter. If a variable

name contains multiple words, usually an underscore is used to join the two words,

or the first letter of the second word is uppercase. So you would write my_variable

or myVariable.

TABLE 17.2 JavaScript Reserved Words
abstract final return

arguments finally short

boolean float static

break for super

byte function switch

case goto synchronized

catch if this

char implements throw

class import throws

const in transient

continue instanceof true

debugger int try

default interface typeof

delete let var