JavaScript features a couple of methods that lets you run a piece of JavaScript code (javascript function) at some point in the future. These methods are:

• setTimeout()
• setInterval()

In this tutorial, I'll explain how setInterval() method works, and give a real world example. You may find the details of setTimeout() method in JavaScript setTimeout Function - JavaScript Timing Events

setInterval()

window.setInterval() method allows you to specify a piece of JavaScript code (expression) that will be triggered again and again in every specified number of miliseconds.

Syntax

var t = setInterval ( expression, timeout );

The setInterval() method returns a numeric timeout ID which can be used to refer the time interval to use with clearInterval method. The first parameter (expression) of setInterval() is a string containing a javascript statement. The statement could be a call to a JavaScript function like "delayedAlert();" or a  statement like "alert('This alert is delayed.');". The second parameter (timeout), indicates the number of miliseconds to pass before executing the expression.

Example 

<script type="text/javascript">
     setInterval("doSomething()", 5000);

     function doSomething()
     {
           // do something        
     }
</script>
  

The doSomething() function will be called in every 5 seconds until it is cancelled.

clearInterval()

If you want to cancel a setInterval() then you need to call clearInterval(), passing the internal ID returned by the call to setInterval(). 

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