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2.4 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 iden-
tifier must begin with a letter, an underscore (_), or a dollar sign ($). Subsequent char-
acters 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:

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.

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