Handle loading of images and videos with promises

export default function(elements) {
  return new Promise((resolve, reject) => {
    let elementsLoaded = 0;
    const isFinishedLoading = (a) => {
      if (elementsLoaded === elements.length) {
        resolve();
      }
    };

    elementsLoaded = 0;
    elements.forEach(el => {
      if(el.nodeName === 'IMG') {
        if (el.complete) {
          elementsLoaded++;
        }
        else {
          el.addEventListener('load', () => {
            elementsLoaded++;
            isFinishedLoading();
          });
        }
      }

      if(el.nodeName === 'VIDEO') {
        inlineVideo(el, false);
        if(el.readyState === 4) {
            elementsLoaded++;
            isFinishedLoading();
        } else {
          el.addEventListener('canplay', () => {
            elementsLoaded++;
            isFinishedLoading();
          });
        }
      }
      isFinishedLoading();
    });
  });
  }