Javascript [Electronic resources] : The Definitive Guide (4th Edition) نسخه متنی

2.4 Optional Semicolons

Simple statements in JavaScript are
generally followed by semicolons (;), just as they
are in C, C++, and Java. The semicolon serves to separate statements
from each other. In JavaScript, however, you may omit the semicolon
if each of your statements is placed on a separate line. For example,
the following code could be written without semicolons:

a = 3;
b = 4;

But when formatted as follows, the first semicolon is required:

a = 3; b = 4;

Omitting semicolons is not a good programming practice; you should
get in the habit of using them.

Although JavaScript
theoretically allows line breaks between any two tokens, the fact
that JavaScript automatically inserts semicolons for you causes some
exceptions to this rule. Loosely, if you break a line of code in such
a way that the line before the break appears to be a complete
statement, JavaScript may think you omitted the semicolon and insert
one for you, altering your meaning. Some places you should look out
for this are with the
return,
break, and continue statements
(which are described in Chapter 6). For example,
consider the following:

return
true;

JavaScript assumes you meant:

return;
true;

However, you probably meant:

return true;

This is something to watch out for -- this code does not cause a
syntax error and will fail in a nonobvious way. A similar problem
occurs if you write:

break
outerloop;

JavaScript inserts a semicolon after the break
keyword, causing a syntax error when it tries to interpret the next
line. For similar reasons, the
++ and -- postfix
operators (see Chapter 5) must always appear on
the same line as the expressions to which they are applied.