JS Timing

JavaScript Timing Events

With JavaScript, it is possible to execute some code after a specified time-interval. This is called timing events.

It's very easy to time events in JavaScript. The two key methods that are used are:

  • setTimeout() - executes a code some time in the future
  • clearTimeout() - cancels the setTimeout()

Note: The setTimeout() and clearTimeout() are both methods of the HTML DOM Window object.

The setTimeout() Method

Syntax

var t=setTimeout("javascript statement",milliseconds);

The setTimeout() method returns a value - In the statement above, the value is stored in a variable called t. If you want to cancel this setTimeout(), you can refer to it using the variable name.

The first parameter of setTimeout() is a string that contains a JavaScript statement. This statement could be a statement like "alert('5 seconds!')" or a call to a function, like "alertMsg()".

The second parameter indicates how many milliseconds from now you want to execute the first parameter.

Note: There are 1000 milliseconds in one second.

Example

When the button is clicked in the example below, an alert box will be displayed after 5 seconds.

Example

<html>
<head>
<script type="text/javascript">
function timedMsg()
{
var t=setTimeout("alert('5 seconds!')",5000);
}
</script>
</head>

<body>
<form>
<input type="button" value="Display timed alertbox!"
onClick="timedMsg()" />
</form>
</body>
</html>

Try it yourself »

Example - Infinite Loop

To get a timer to work in an infinite loop, we must write a function that calls itself.

In the example below, when a button is clicked, the input field will start to count (for ever), starting at 0.

Notice that we also have a function that checks if the timer is already running, to avoid creating additional timers, if the button is pressed more than once:

Example

<html>
<head>
<script type="text/javascript">
var c=0;
var t;
var timer_is_on=0;

function timedCount()
{
document.getElementById('txt').value=c;
c=c+1;
t=setTimeout("timedCount()",1000);
}

function doTimer()
{
if (!timer_is_on)
{
timer_is_on=1;
timedCount();
}
}
</script>
</head>

<body>
<form>
<input type="button" value="Start count!" onClick="doTimer()">
<input type="text" id="txt" />
</form>
</body>
</html>

Try it yourself »

 

The clearTimeout() Method

Syntax

clearTimeout(setTimeout_variable)

Example

The example below is the same as the "Infinite Loop" example above. The only difference is that we have now added a "Stop Count!" button that stops the timer:

Example

<html>
<head>
<script type="text/javascript">
var c=0;
var t;
var timer_is_on=0;

function timedCount()
{
document.getElementById('txt').value=c;
c=c+1;
t=setTimeout("timedCount()",1000);
}

function doTimer()
{
if (!timer_is_on)
{
timer_is_on=1;
timedCount();
}
}

function stopCount()
{
clearTimeout(t);
timer_is_on=0;
}
</script>
</head>

<body>
<form>
<input type="button" value="Start count!" onClick="doTimer()">
<input type="text" id="txt">
<input type="button" value="Stop count!" onClick="stopCount()">
</form>
</body>
</html>