I thought I would try and be clever and create a Wait function of my own (I realise there are other ways to do this). So I wrote:

var interval_id;
var countdowntimer = 0;

function Wait(wait_interval) {
  countdowntimer = wait_interval;

  interval_id = setInterval(function() {
    --countdowntimer <=0 ? clearInterval(interval_id) : null;
  }, 1000);

  do {} while (countdowntimer >= 0);
}

// Wait a bit: 5 secs
Wait(5);

This all works, except for the infinite looping. Upon inspection, if I take the While loop out, the anonymous function is entered 5 times, as expected. So clearly the global variable countdowntimer is decremented.

However, if I check the value of countdowntimer, in the While loop, it never goes down. This is despite the fact that the anonymous function is being called whilst in the While loop!

Clearly, somehow, there are two values of countdowntimer floating around, but why?