tldr: ALWAYS take a brief look at official developer documentation of functions.

I was trying to rush a release over weekend and I had requirement where I was to make repeated API calls to track progress of status of task. Without that any new user would be seeing a "dead page" without any info on what is going on and how he/she should proceed. Pseudo code would be something like:

function get_task_status(task_id) {
  $.get("/get_task_status/", {'task_id':task_id})
    .done( function(data) {
      // Update status div
      // Wait for x seconds and repeat this function
    });
}

As usual I hurried to Google search the template/pointer code. StackOverflow didn't disappoint and I landed up with this discussion. It had decent insight on using callback function with setTimeout and I cooked up my own version of it:

function get_task_status(task_id) {
  $.get("/get_task_status/", {'task_id':task_id})
    .done( function(data) {
      if(data['task'] === 'PROGRESS') {
	Materialize.toast(data['info'], 1000);
	setTimeout(get_task_status(task_id), 2000);
      }
    });
}

Looks innocent right? Well that's what got me stumped for almost 3-4 hours. I tried this and my javascript happily ignored setTimeout and delay in seconds and kept making continues GET requests. I tried some variants of above code but nothing worked. Eventually I came across this post on SO, tried the code and it worked! I was convinced that there was some issue with older version handling of setTimeout and 2016 update is what I needed.

Today as I sat to put together a brief note of this experience I was testing setTimeout code on node, browser console, inside HTML template and somehow each time delay was working just fine:

function get_task_status() {
  console.log(Date());
  setTimeout(get_task_status, 2000);
}

> function get_task_status(task_id) {
...   console.log(Date());
...   // Recursive call to this function itself after 2 seconds of delay
...   setTimeout(get_task_status, 2000);
... }
undefined
> get_task_status('something');
Tue Oct 25 2016 15:55:59 GMT+0530 (IST)
undefined
> Tue Oct 25 2016 15:56:01 GMT+0530 (IST)
Tue Oct 25 2016 15:56:03 GMT+0530 (IST)
Tue Oct 25 2016 15:56:05 GMT+0530 (IST)
Tue Oct 25 2016 15:56:07 GMT+0530 (IST)
Tue Oct 25 2016 15:56:09 GMT+0530 (IST)
(To exit, press ^C again or type .exit)

Again, this bummed me, I thought I had "established" that setTimeout was broken and promise is what I should be looking at and get better understanding of. While trying to work it out and understand what is wrong as I checked MDN documentation of the function and I finally realized my real bug. Syntax of function is var timeoutID = window.setTimeout(func[, delay, param1, param2, ...]);

And this is what I was doing: setTimeout(get_task_status(task_id), 2000);

Notice in syntax params are after the delay argument while I just used them directly and this was the small gotcha. I was talking to Syed ji about this experience and he pointed to You don't know series for better understanding of javascript concepts and nuances. I learned my lesson, properly RTFM and as for promise, I will return to learn more about it later, at the moment my code is working.