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

6.2 Compound Statements

Chapter 5, we saw that the
comma operator can be used to combine a
number of
expressions into a single
expression. JavaScript also has a way to combine a number of
statements into a single statement, or

statement
block . This is done simply by enclosing any number of
statements within curly braces. Thus, the following lines act as a
single statement and can be used anywhere that JavaScript expects a
single statement:

{
x = Math.PI;
cx = Math.cos(x);
alert("cos(" + x + ") = " + cx);
}

Note that although this statement block acts as a single statement,
it does

not end with a semicolon. The primitive
statements within the block end in semicolons, but the block itself
does not.

Although combining expressions with the comma operator is an
infrequently used technique, combining statements into larger
statement blocks is extremely common. As we'll see in the
following sections, a number of JavaScript statements themselves
contain statements (just as expressions can contain other
expressions); these statements are

compound
statements . Formal JavaScript syntax specifies that each
of these compound statements contains a single substatement. Using
statement blocks, you can place any number of statements within this
single allowed substatement.

To execute a compound statement, the JavaScript interpreter simply
executes the statements that comprise it one after another, in the
order in which they are written. Normally, the JavaScript interpreter
executes all of the statements. In some circumstances, however, a
compound statement may terminate abruptly. This termination occurs if
the compound statement contains a
break
, continue,
return, or throw statement, if
it causes an error, or if it calls a function that causes an uncaught
error or throws an uncaught exception. We'll learn more about
these abrupt terminations in later sections.