fitelement(something with aspect), script for rAF.js

fitelement.js

var fitElement = function fitElement(elementSelector, elementAspect) {
  var element = (typeof elementSelector === 'string' || elementSelector instanceof String) ? document.querySelector(elementSelector) : elementSelector,
    parW = element.parentNode.clientWidth,
    parH = element.parentNode.clientHeight,
    parAsp = parW / parH,
    aspect = elementAspect || 16 / 9,
    elW, elH;
  if (aspect > parAsp) {
    elW = parW;
    elH = elW / aspect;
  } else {
    elH = parH;
    elW = elH * aspect;
  }
  //only if changed on frame
  if (element.clientHeight !== parseInt(elH.toFixed())) {
    element.style.height = elH.toFixed() + 'px';
  }
  if (element.clientWidth !== parseInt(elW.toFixed())) {
    element.style.width = elW.toFixed() + 'px';
  }
};

var elementObj = document.querySelector('.element-class');
(function animloop() {
  fitElement(elementObj);
  requestAnimationFrame(animloop);
})();