javascript

Chapter 8: The Browser Object Model
When setTimeout() is called, it returns a numeric ID for the timeout. The timeout ID is a unique
identifier for the scheduled code that can be used to cancel the timeout. To cancel a pending timeout, use
the clearTimeout() method and pass in the timeout ID as in the following example:
//set the timeout
var timeoutId = setTimeout(function() {
alert(“Hello world!”);
}, 1000);
//nevermind - cancel it
clearTimeout(timeoutId);
As long as clearTimeout() is called before the specified amount of time has passed, a timeout can be
canceled completely. Calling clearTimeout() after the code has been executed has no effect.
All code executed by a timeout runs in the global scope, so the value of this inside the function will
always point to window .
Intervals work in the same way as timeouts except that they execute the code repeatedly at specific time
intervals until the interval is canceled or the page is unloaded. The setInterval() method lets you set
up intervals, and it accepts the same arguments as setTimeout() : the code to execute (string or
function) and the amount of time in milliseconds to wait between executions. Here ’ s an example:
//avoid!
setInterval(“alert(‘Hello world!’) “, 10000);
//preferred
setInterval(function() {
alert(“Hello world!”);
}, 10000);
The setInterval() method also returns an interval ID that can be used to cancel the interval at some
point in the future. The clearInterval() method can be used with this ID to cancel all pending
intervals. This ability is more important for intervals than timeouts since, if left unchecked, they continue
to execute until the page is unloaded. Here is a common example of interval usage:
var num = 0;
var max = 10;
var intervalId = null;
function incrementNumber() {
num++;
}
//if the max has been reached, cancel all pending executions
if (num == max) {
clearInterval(intervalId);
alert(“Done”);
}
intervalId = setInterval(incrementNumber, 500);
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