Execute script after specific delay using JavaScript

Question

Just to add to what everyone else have said about setTimeout: If you want to call a function with a parameter in the future, you need to set up some anonymous function calls.

You need to pass the function as an argument for it to be called later. In effect this means without brackets behind the name. The following will call the alert at once, and it will display 'Hello world':

var a = "world";
setTimeout(alert("Hello " + a), 2000);

To fix this you can either put the name of a function (as Flubba has done) or you can use an anonymous function. If you need to pass a parameter, then you have to use an anonymous function.

var a = "world";
setTimeout(function(){alert("Hello " + a)}, 2000);
a = "Stack Overflow";

But if you run that code you will notice that after 2 seconds the popup will say 'Hello Stack Overflow'. This is because the value of the variable a has changed in those two seconds. To get it to say 'Hello world' after two seconds, you need to use the following code snippet:

function callback(a){
    return function(){
        alert("Hello " + a);
    }
}
var a = "world";
setTimeout(callback(a), 2000);
a = "Stack Overflow";

It will wait 2 seconds and then popup 'Hello world'.

There is the following:

setTimeout(function, milliseconds);

function which can be passed the time after which the function will be executed.

See: Window setTimeout() Method.

Just to expand a little... You can execute code directly in the setTimeout call, but as @patrick says, you normally assign a callback function, like this. The time is milliseconds

setTimeout(func, 4000);
function func() {
    alert('Do stuff here');
}

If you really want to have a blocking (synchronous) delay function (for whatsoever), why not do something like this:

<script type="text/javascript">
    function delay(ms) {
        var cur_d = new Date();
        var cur_ticks = cur_d.getTime();
        var ms_passed = 0;
        while(ms_passed < ms) {
            var d = new Date();  // Possible memory leak?
            var ticks = d.getTime();
            ms_passed = ticks - cur_ticks;
            // d = null;  // Prevent memory leak?
        }
    }

    alert("2 sec delay")
    delay(2000);
    alert("done ... 500 ms delay")
    delay(500);
    alert("done");
</script>

You need to use setTimeout and pass it a callback function. The reason you can't use sleep in javascript is because you'd block the entire page from doing anything in the meantime. Not a good plan. Use Javascript's event model and stay happy. Don't fight it!

You can also use window.setInterval() to run some code repeatedly at a regular interval.

To add on the earlier comments, I would like to say the following :

The setTimeout() function in JavaScript does not pause execution of the script per se, but merely tells the compiler to execute the code sometime in the future.

There isn't a function that can actually pause execution built into JavaScript. However, you can write your own function that does something like an unconditional loop till the time is reached by using the Date() function and adding the time interval you need.