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

6.11 break

The break


statement causes the innermost
enclosing loop or a switch statement to exit
immediately. Its syntax is simple:

break;

Because it causes a loop or switch to exit, this
form of the break statement is legal only if it
appears within one of these statements.

ECMAScript v3 and JavaScript 1.2 allow the break
keyword
to be followed by the name of a label:

break labelname;

Note that labelname is simply an
identifier; it is not followed by a colon, as it would be when
defining a labeled statement.

When break is used with a label, it jumps to the
end of, or terminates, the named statement, which may be any
enclosing statement. The named statement need not be a loop or
switch; a break statement used
with a label need not even be contained within a loop or
switch. The only restriction on the label of the
break statement is that it name an

enclosing statement. The label can name an
if statement, for example, or even a block of
statements grouped within curly braces, for the sole purpose of
naming the block with a label.

Chapter 2, a newline is not allowed between the
break keyword and the
labelname. This is an oddity of JavaScript
syntax caused by its automatic insertion of omitted semicolons. If
you break a line of code between the break keyword
and the following label, JavaScript assumes you meant to use the
simple, unlabeled form of the statement and adds a semicolon for you.

We've already seen examples of the break
statement within a switch statement. In loops, it
is typically used to exit prematurely when, for whatever reason,
there is no longer any need to complete the loop. When a loop has
complex termination conditions, it is often easier to implement some
of these conditions with break statements, rather
than trying to express them all in a single loop expression.

The following code searches the elements of an array for a particular
value. The loop terminates naturally when it reaches the end of the
array; it terminates with a break statement if it
finds what it is looking for in the array:

for(i = 0; i < a.length; i++) {
if (a[i] == target)
break;
}

You need the labeled form of the break statement
only when you are using nested loops or switch
statements and need to break out of a statement that is not the
innermost one.

The following example shows
labeled for loops and labeled
break statements. See if you can figure out what
its output will be:

outerloop:
for(var i = 0; i < 10; i++) {
innerloop:
for(var j = 0; j < 10; j++) {
if (j > 3) break;             // Quit the innermost loop
if (i == 2) break innerloop;  // Do the same thing
if (i == 4) break outerloop;  // Quit the outer loop
document.write("i = " + i + " j = " + j + "<br>");
}
}
document.write("FINAL i = " + i + " j = " + j + "<br>");