What does while(i--) mean in javascript?

The number 0 is considered falsey in JavaScript, so the while loop will run until it hits 0. You would have to be sure that i is positive to start off with, otherwise you've got an infinite loop.

var i = bullets.length;
while (i--) {
    ...do stuff...
}

is essentially equivalent to:

var i = bullets.length;
while (true) {
    if (i === 0) {
        i = i - 1;
        break;
    }
    i = i - 1;
    ...do stuff...
}

This style of looping tends to be used where performance is super important, as it's slightly faster than iterating indices from 0 to length-1, and is relatively straight-forward to implement.

Generally speaking, you shouldn't use it unless you're improving a known bottleneck and have verified that it improves performance in a significant way.

It will first check i value (if not zero), then enter loop after having decremented i. So, if i = 10, loop will be executed with i = 9, i = 8, .., i = 0 and then exit (because in last i-- check, i will be already zero from previous loop run)