Loops execute a block of code a specified number of times, or while a specified condition is true.


The while Loop

The while loop loops through a block of code while a specified condition is true.

Syntax
Code:
while (variable<=endvalue)
  {
      code to be executed
  }
Note: The <= could be any comparing operator.



The example below defines a loop that starts with i=0. The loop will continue to run as long as i is less than, or equal to 5. i will increase by 1 each time the loop runs:

Example
Code:
<html>
<body>
<script type="text/javascript">
var i=0;
while (i<=5)
  {
  document.write("The number is " + i);
  document.write("<br />");
  i++;
  }
</script>
</body>
</html>
Try it yourself


The do...while Loop

The do...while loop is a variant of the while loop. This loop will execute the block of code ONCE, and then it will repeat the loop as long as the specified condition is true.

Syntax

Code:
do
  {
      code to be executed
  }
while (variable<=endvalue);
The example below uses a do...while loop. The do...while loop will always be executed at least once, even if the condition is false, because the statements are executed before the condition is tested:

Example
Code:
<html>
<body>
<script type="text/javascript">
var i=0;
do
  {
  document.write("The number is " + i);
  document.write("<br />");
  i++;
  }
while (i<=5);
</script>
</body>
</html>
Try it yourself