Identifiers (JavaScript: The Definitive Guide, 4th Edition)

2.7. Identifiers

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. The rules for legal identifier names are the same in JavaScript as they are in Java and many other languages. The first character must be a letter, an underscore (_), or a dollar sign ($).[3] Subsequent characters may be any letter or digit or an underscore or dollar sign. (Numbers are not allowed as the first character so that JavaScript can easily distinguish identifiers from numbers.) These are all legal identifiers:

[3]Note that dollar signs are not legal in identifiers prior to JavaScript 1.1. They are intended for use only by code-generation tools, so you should avoid using dollar signs in identifiers in the code you write yourself.

i
my_variable_name
v13
_dummy
$str

In ECMAScript v3, identifiers can contain letters and digits from the complete Unicode character set. Prior to this version of the standard, JavaScript identifiers are restricted to the ASCII character set. ECMAScript v3 also allows Unicode escape sequences to appear in identifiers. A Unicode escape is the characters \u followed by 4 hexadecimal digits that specify a 16-bit character encoding. For example, the identifier could also be written as \u03c0. Although this is an awkward syntax, it makes it possible to translate JavaScript programs that contain Unicode characters into a form that allows them to be manipulated with text editors and other tools that do not support the full Unicode character set.

Finally, identifiers cannot be the same as any of the keywords used for other purposes in JavaScript. The next section lists the special names that are reserved in JavaScript.