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


JavaScript Loops

Often when you write code, you want the same block of code to run over and over again in a row. Instead of adding several almost equal lines in a script we can use loops to perform a task like this.

In JavaScript, there are two different kind of loops:

  • for - loops through a block of code a specified number of times
  • while - loops through a block of code while a specified condition is true


The for Loop

The for loop is used when you know in advance how many times the script should run.

Syntax
Code:
for (variable=startvalue;variable<=endvalue;variable=variable+increment)
{
    code to be executed
}
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.

Note: The increment parameter could also be negative, and the <= could be any comparing statement.

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