Checks for an elements existence within a RAF within a promise.

elementReady.js

/**
 * Checks for an elements existence within a RAF within a promise.
 *
 * @param {string} selector - The element you wish to find. Defaults to 'body'.
 * @param {string} target - The parent element to search within. Defaults to document.
 * @return {Promise} Resolves when the element exists.
 */
function elementReady(selector, target) {
  var options = {
    target: target || document,
    selector: selector || 'body'
  }
  return new Promise(function promise(resolve) {
    (function check() {
      var el = options.target.querySelector(options.selector);
      if (el) {
        resolve(el);
      }
      requestAnimationFrame(check);
    })();
  });
}