polling-deferred.js

1-polling-deferred.js

// The polling function
function poll(fn, timeout, interval) {
    var dfd = new Deferred();
    var endTime = Number(new Date()) + (timeout || 2000);
    interval = interval || 100;

    (function p() {
            // If the condition is met, we're done! 
            if(fn()) {
                dfd.resolve();
            }
            // 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 {
                dfd.reject(new Error('timed out for ' + fn + ': ' + arguments));
            }
    })();

    return dfd.promise;
}

// Usage:  ensure element is visible
poll(function() {
	return document.getElementById('lightbox').offsetWidth > 0;
}, 2000, 150);

2-Description.md

With Deferreds: Most JavaScript frameworks/toolkits have a Deferred/Promise implementation so it's implementation would revolve around the Deferred. The code is structured easy enough to read but it's mostly three-fold: the conditional function which signals polling success, a conditional failure which hasn't timeout out, so we'll run again, or a failure which has run past timeout which should return an error.Date.now() vs Number(new Date())

http://davidwalsh.name/javascript-polling