Is setTimeout with no delay the same as executing the function instantly?

Question

I am looking at some existing code in a web application. I saw this:

window.setTimeout(function () { ... })

Is this the same as just executing the function content right away?

Answer 1

It won't necessarily run right away, neither will explicitly setting the delay to 0. The reason is that setTimeout removes the function from the execution queue and it will only be invoked after JavaScript has finished with the current execution queue.

console.log(1);
setTimeout(function() {console.log(2)});
console.log(3);
console.log(4);
console.log(5);
//console logs 1,3,4,5,2

for more details see http://javascriptweblog.wordpress.com/2010/06/28/understanding-javascript-timers/

Answer 2

There is a minimum delay that setTimeout uses (4ms as per HTML5, Firefox 3.6 uses 10ms). There is a discussion about it on the Mozilla Developer Center documentation page.

Answer 3

You are missing the millisecond parameter...

setTimeout(function() { /*something*/ }, 0);

The 0 sets the delay to 0 but what it actually does is to let your function "jump the queue" of the browser execution list. The browser has a bunch of things to do such as rendering objects on the page, and by calling this, your function will run as soon as the browser has some cycles.