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
$strFor 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 throw while
debugger finally new true with
default for null tryJavaScript also reserves certain keywords that are not
currently used by the language but which might be used in future
versions. ECMAScript 5 reserves the following words:
class const enum export extends import superIn addition, the following words, which are legal in ordinary
JavaScript code, are reserved in strict mode:
implements let private public yield
interface package protected staticStrict mode also imposes restrictions on the use of the
following identifiers. They are not fully reserved, but they are not
allowed as variable, function, or parameter names:
arguments evalECMAScript 3 reserved all the keywords of the Java language,
and although this has been relaxed in ECMAScript 5, you should still
avoid all of these identifiers if you plan to run your code under an
ECMAScript 3 implementation of JavaScript:
abstract double goto native static
boolean enum implements package super
byte export import private synchronized
char extends int protected throws
class final interface public transient
const float long short volatileJavaScript predefines a number of global variables and
functions, and you should avoid using their names for your own
variables and functions:
arguments encodeURI Infinity Number RegExp
Array encodeURIComponent isFinite Object String
Boolean Error isNaN parseFloat SyntaxError
Date eval JSON parseInt TypeError
decodeURI EvalError Math RangeError undefined
decodeURIComponent Function NaN ReferenceError URIErrorKeep in mind that JavaScript implementations may define other
global variables and functions, and each specific JavaScript
embedding (client-side, server-side, etc.) will have its own list of
global properties. See the Window object in Part IV
for a list of the global variables and functions defined by
client-side JavaScript.