JavaScript setTimeout() won't wait to Execute?

Question

Consider the following example:

<script type="text/javascript">
    function alertBox(){
        alert('Hello World!');
    }
    function doSomething(){
        setInterval(alertBox(), 5000); //This is for generic purposes only
    };
    function myFunction(){
        setTimeout(doSomething(),3000);
    };

    myFunction();
</script>

What is it that causes this to execute IMMEDIATELY, rather than waiting the 3 seconds set, as well as only executing the alert ONCE, rather than at the scheduled 5 second intervals?

Thanks for any help you can provide!

Mason

Well, **the callback is invoked immediately** (and the *result* is passed to `setTimeout`) .. pass the Function-object instead. — , Aug 07 '12 at 00:00
This is probably the single most asked javascript question on SO. — jfriend00, Aug 07 '12 at 00:01
@jfriend00 I think closured variables in loops might tie it .. ;-) — , Aug 07 '12 at 00:01
Well I certainly feel like a fool - This is what I get for leaving JS alone for so long >. — MasonWinsauer, Aug 07 '12 at 00:03
Refer to my comment, it might useful for you.https://stackoverflow.com/a/60630117/4248767 — Nuwa, Mar 11 '20 at 06:11

score 21 · Accepted Answer · answered Aug 07 '12 at 00:00

21

alertBox()

Doesn't this look like an immediate function call?

Try passing the function (without executing it) instead:

setInterval(alertBox, 5000);

answered Aug 07 '12 at 00:00

Ates Goral

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hvgotcodes · Answer 2 · 2012-08-07T00:10:41.283

14

its because you are executing the function, not passing a function object.

function myFunction(){
    setTimeout(doSomething, 3000); // no () on the function
};

edited Aug 07 '12 at 00:10

answered Aug 07 '12 at 00:00

hvgotcodes

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score 0 · Answer 3 · answered Mar 11 '20 at 06:05

Following codes executing differently,

setTimeout(console.log('Nuwan'),0)---(A); and 
setTimeout(function(){console.log('Nuwan'),0}) --- (B)

The reason is when (B) is executing, CallStack in javascript runtime push the setTimeout() to web API and WebAPI waits 0 seconds to push the callback function to the event queue and then EventLoop pushes the callback to the stack after the current stack is empty.

In Case (A), console.log() directly calling because it is in the WepAPI, no need to go for an event loop to execute.

More Ref: WebAPIs: https://developer.mozilla.org/en-US/docs/Web/API/Console_API Javascript Runtime and Event Loop: https://cybrohosting.com/knowledgebase/17/How-JavaScript-works-in-browser-and-node.html

JavaScript setTimeout() won't wait to Execute?

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