Question

192

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

rated 0 times [ 197] [ 5] / answers: 1 / hits: 15082 / 10 Years ago, tue, march 4, 2014, 12:00:00

I'm using swipejs from swipejs.com, the home page slider uses this logic to highlight the active un-ordered list element and I'd like to know how this while condition works.

<ul id=bullets>

   <li></li>

    ....

</ul>



var bullets = document.getElementById('bullets').getElementsByTagName('li');



var elem = document.getElementById('mySwipe');



window.mySwipe = Swipe(elem, {



callback: function(index, element) {

    var i = bullets.length;

    while (i--) {

      bullets[i].className = ' ';

    }

    bullets[index].className = 'on';

  }



});

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janettejordynm

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answered 10 Years ago anyssaarielles · Accepted Answer

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.