Javascript Callback timeout

Question

I have a fiddle here

Can someone help me understand why the first setTimeout works but not on second one? Please see comments on the code.

In this case, I want to alert first I am first then after 6 seconds, it would alert Hello, sorry I am late

function iAmTakingTooLong(message1, message2, callback){       
    //setTimeout(function(){ alert('test'); },6000); //THIS WILL WAIT FOR 6000 MILLISECONDS
    setTimeout(callback(message1+message2),6000);    //THIS WILL NOT WAIT FOR 6000 MILLISECONDS
}


iAmTakingTooLong('Hello, ', 'sorry I am late!', function(fullmessage){
    alert(fullmessage);
});

alert("I am first!");

Pointy · Accepted Answer · 2014-03-27 14:36:17Z

11

In this code:

setTimeout(callback(message1+message2),6000);

you're calling the callback function right there in the argument list. JavaScript evaluates function arguments before calling the function, so what actually gets passed to setTimeout() here is whatever the callback function returns.

You need something like:

setTimeout(function() { callback(message1 + message2); }, 6000);

The behavior here is not unique to JavaScript.

answered Mar 27, 2014 at 14:36

Pointy

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3 Comments

JunM Over a year ago

Thanks man, I am just wondering since I was thinking that the callback is already a function and doesn't need to put it inside function(){}

Pointy Over a year ago

What would you expect callback(message1 + message2) to do outside of a call to setTimeout()?

JunM Over a year ago

I am expecting that without putting callback(message1+message2) inside function(){}, it would call the callback only after the specified time. But it seems that is not the case. Thank you.

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