polling.js

1-polling.js

function poll(fn, callback, errback, timeout, interval) {
    var endTime = Number(new Date()) + (timeout || 2000);
    interval = interval || 100;

    (function p() {
            // If the condition is met, we're done! 
            if(fn()) {
                callback();
            }
            // If the condition isn't met but the timeout hasn't elapsed, go again
            else if (Number(new Date()) < endTime) {
                setTimeout(p, interval);
            }
            // Didn't match and too much time, reject!
            else {
                errback(new Error('timed out for ' + fn + ': ' + arguments));
            }
    })();
}

// Usage:  ensure element is visible
poll(
    function() {
        return document.getElementById('lightbox').offsetWidth > 0;
    },
    function() {
        // Done, success callback
    },
    function() {
        // Error, failure callback
    }
);

2-Description.md

If you aren't using Deferreds, no worry -- polling is just about the same. The difference here is that there's no return value -- the callback functions take the place of the Deferred instance.Polling isn't necessarily a consequence of async coding but it has definitely increased in usage and importance due to our desire to write async code. During my time writing front-end functional tests with the Intern testing framework, I've used polling quite a bit both on the server and client sides. This technique will always have its place so make sure you have a snippet like this available.

http://davidwalsh.name/javascript-polling

3-polling-comments.md

Andrea Giammarchi I would return {cancel: function () { clearTimeout(interval); }} to the non deferred option in order to provide superior control to the poller author.

David Walsh That’s a great idea!

Date.now() vs Number(new Date())