JavaScript: The Definitive Guide, 6th Edition

Identifiers and Reserved Words

An identifier is simply a name. In JavaScript, identifiers are used to name variables and functions and to provide labels for certain loops in JavaScript code. A JavaScript identifier must begin with a letter, an underscore (_), or a dollar sign ($). Subsequent characters can be letters, digits, underscores, or dollar signs. (Digits are not allowed as the first character so that JavaScript can easily distinguish identifiers from numbers.) These are all legal identifiers:

i
my_variable_name
v13
_dummy
$str

For portability and ease of editing, it is common to use only ASCII letters and digits in identifiers. Note, however, that JavaScript allows identifiers to contain letters and digits from the entire Unicode character set. (Technically, the ECMAScript standard also allows Unicode characters from the obscure categories Mn, Mc, and Pc to appear in identifiers after the first character.) This allows programmers to use variable names from non-English languages and also to use mathematical symbols:

var sí = true;
var π = 3.14;

Like any language, JavaScript reserves certain identifiers for use by the language itself. These “reserved words” cannot be used as regular identifiers. They are listed below.

Reserved Words

JavaScript reserves a number of identifiers as the keywords of the language itself. You cannot use these words as identifiers in your programs:

break delete function return typeof case do if switch var catch else in this void continue false instanceof ...