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

6.8 for

The
for statement provides a looping construct that is
often more convenient than the while statement.
The for statement takes advantage of a pattern
common to most loops (including the earlier while
loop example). Most loops have a counter variable of some kind. This
variable is initialized before the loop starts and is tested as part
of the expression evaluated before each
iteration of the loop. Finally, the counter variable is incremented
or otherwise updated at the end of the loop body, just before
expression is evaluated again.

The
initialization, the test, and the
update are the three crucial manipulations of a loop
variable; the
for statement makes these three steps an explicit
part of the loop syntax. This makes it especially easy to understand
what a for loop is doing and prevents mistakes
such as forgetting to initialize or increment the loop variable. The
syntax of the for statement is:

for(initialize ; test ; increment)
statement

The simplest way to explain what this for loop
does is to show the equivalent while
loop:[3]

[3] As we will see when we consider the
continue statement, this while
loop is not an exact equivalent to the for loop.

initialize;
while(test) {
statement
increment;
}

In other words, the initialize expression
is evaluated once, before the loop begins. To be useful, this is an
expression with side effects (usually an assignment). JavaScript also
allows initialize to be a
var variable declaration statement, so that you
can declare and initialize a loop counter
at the same time. The test expression is
evaluated before each iteration and controls whether the body of the
loop is executed. If the test expression
is true, the statement
that is the body of the loop is executed. Finally, the
increment expression is evaluated. Again,
this must be an expression with side effects in order to be useful.
Generally, either it is an assignment expression or it uses
the ++ or
-- operators.

The example while loop of the previous section can
be rewritten as the following for loop, which
counts from 0 to 9:

for(var count = 0 ; count < 10 ; count++)
document.write(count + "<br>");

Notice that this syntax places all the important information about
the loop variable on a single line, which makes it clear how the loop
executes. Also note that placing the
increment expression in the
for statement itself simplifies the body of the
loop to a single statement; we don't even need to use curly
braces to produce a statement block.

Loops can become
a lot more complex than these simple examples, of course, and
sometimes multiple variables change with each iteration of the loop.
This situation is the only place that the comma operator is commonly
used in JavaScript -- it provides a way to combine multiple
initialization and increment expressions into a single expression
suitable for use in a for loop. For example:

for(i = 0, j = 10 ; i < 10 ; i++, j--)
sum += i * j;