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
and the process continues until such a time as the expression
evaluates to false
, or until
execution is otherwise halted. Example 15-17
shows such a 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 may not even be easy to terminate with Escape or the Stop button. So 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 15-18
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