Are equal timeouts executed in order in Javascript?

Question

Suppose I do

setTimeout(foo, 0);

...

setTimeout(bar, 0);

Can I be sure foo will begin executing before bar? What if instead of 0 I use a

Answer 1

The answer is: No, execution order is not guaranteed. I have occasionally seen non-FIFO ordering in chrome 40, especially if I pause in debugger while timeouts are pending. I also had a rare sighting in the wild - it was the only plausible explanation of an incident I investigated.

If you need guaranteed execution order, instead of setTimeout(callback, 0) you can do this:

var queue = [];
function handleQueue() {
    queue.shift()();
}
function queueCallback(callback) {
   queue.push(callback);
   setTimeout(handleQueue, 0);
}

Now, for guaranteed foo, bar order you can:

queueCallback(foo);
...
queueCallback(bar);