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.
<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.
doâ¦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.
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