setTimeout proper syntax

May 3, 2011 javascript

This post is more than 18 months old. Since technology changes too rapidly, this content may be out of date (but that's not always the case). Please remember to verify any technical or programming information with the current release.

The javascript function setTimeout can be used to execute a function after a specified amount of delay. It uses the javascript callback pattern. Its important to use the callback pattern correctly, however, or you may get different results than you expect. One example of this is the immediate execution of a function versus the callback execution. Note this example:

function myTime(x)
{
  alert(x);
  alert(new Date());
}
alert(new Date());

Incorrect usage:

setTimeout(myTime(), 5000);

When this is run, the script executes an alert that shows the current time. After this is dismissed, immediately the myTime() function is executed and another alert shows the current time.

The proper way to do this is to make sure the function is not called immediately, and is referenced in callback pattern:

setTimeout(myTime, 5000);

Now, the current date is immediately alerted. Five seconds later, the next date is alerted.

Last note: some have been known to enclose the function call in string notation:

setTimeout("myTime()", 5000);

This is considered to be bad form as the string must be evaluated and it is not a standard function callback pattern.