js代码

<script>
function castParallax() {

  var opThresh = 350;
  var opFactor = 750;

  window.addEventListener("scroll", function(event) {

    var top = this.pageYOffset;

    var layers = document.getElementsByClassName("parallax");
    var layer, speed, yPos;
    for (var i = 0; i < layers.length; i++) {
      layer = layers[i];
      speed = layer.getAttribute('data-speed');
      var yPos = -(top * speed / 100);
      layer.setAttribute('style', 'transform: translate3d(0px, ' + yPos + 'px, 0px)');

    }
  });
}

document.body.onload = castParallax();
</script>