poll promise

poll-promise.js

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

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

poll(function() {
  return document.getElementById('lightbox').offsetWidth > 0;
}).then(function() {
  // Done
}).catch(function() {
  // Fail
});