vue-material-kit/src/assets/js/plugins/parallax.min.js

var windowHeight = window.innerHeight;

document.addEventListener("resize", function () {
  windowHeight = window.innerHeight;
});

function outerHeight(el) {
  var height = el.offsetHeight;
  var style = getComputedStyle(el);

  height += parseInt(style.marginTop) + parseInt(style.marginBottom);
  return height;
}

function parallax(el, speedFactor, outerHeight) {
  var foo = document.querySelectorAll(el);

  var getHeight;
  var firstTop;
  var paddingTop = 0;

  //get the starting position of each element to have parallax applied to it
  foo.forEach(function (subEl) {
    firstTop = subEl.getBoundingClientRect().top;
  });

  if (outerHeight) {
    getHeight = function (el) {
      return outerHeight(el);
    };
  } else {
    getHeight = function (el) {
      return el.clientHeight;
    };
  }

  // function to be called whenever the window is scrolled or resized
  function update() {
    var pos = window.scrollY;

    foo.forEach(function (subEl) {
      var element = subEl;
      var top = element.getBoundingClientRect().top;
      var height = getHeight(element);

      element.style.top = -Math.round((firstTop - pos) * speedFactor) + "px";
    });
  }
  document.addEventListener("scroll", update, true);
  document.addEventListener("resize", update);
  update();
}

parallax(".hero", -0.6);