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

6.7 do/while

The do/while


loop is much like a while loop, except that the
loop expression is tested at the bottom of the loop rather than at
the top. This means that the body of the loop is always executed at
least once. The syntax is:

do
statement
while (expression);

The do/while statement is implemented in
JavaScript 1.2 and later and standardized by ECMAScript v3.

The do/while loop is less commonly used than its
while cousin. This is because, in practice, it is
somewhat uncommon to encounter a situation in which you are always
sure that you want a loop to execute at least once. For example:

function printArray(a) {
if (a.length == 0)
document.write("Empty Array");
else {
var i = 0;
do {
document.write(a[i] + "<br>");
} while (++i < a.length);
}
} 

There are a couple of differences between the
do/while loop and the ordinary
while loop. First, the do loop
requires both the do

keyword
(to mark the beginning of the loop) and the while
keyword (to mark the end and introduce the loop condition). Also,
unlike the while loop, the do
loop is terminated with a semicolon. This is because the
do loop ends with the loop condition, rather than
simply with a curly brace that marks the end of the loop body.

In JavaScript 1.2, there is a bug in the
behavior of the continue statement (see Section 6.12) when it is used inside a
do/while loop. For this reason, you should avoid
the use of continue within
do/while statements in JavaScript 1.2.