Learning PHP, MySQL, JavaScript, and CSS, 2nd Edition

Looping

Again, you will find many close similarities between JavaScript and PHP when it comes to looping. Both languages support while, do...while, and for loops.

while Loops

A JavaScript while loop first checks the value of an expression and starts executing the statements within the loop only if that expression is true. If it is false, execution skips over to the next JavaScript statement (if any).

Upon completing an iteration of the loop, the expression is again tested to see if it is true; the process continues until such a time as the expression evaluates to false, or execution is otherwise halted. Example 14-17 shows such a loop.

Example 14-17. A while loop

<script>
counter=0

while (counter < 5)
{
    document.write("Counter: " + counter + "<br />")
    ++counter
}
</script>

This script outputs the following:

Counter: 0
Counter: 1
Counter: 2
Counter: 3
Counter: 4

Warning

If the variable counter were not incremented within the loop, it is quite possible that some browsers could become unresponsive due to a never-ending loop, and the page might not even be easy to terminate with Escape or the Stop button. Be careful with your JavaScript loops.

do…while Loops

When you require a loop to iterate at least once before any tests are made, use a do...while loop, which is similar to a while loop except that the test expression is checked only after each iteration of the loop. So, to output the first seven results in the seven times table, you could use code such as that in Example 14-18.

Example 14-18. A ...