scroll-polyfill

scroll-polyfill

(function ScrollPolyfill() {

  // The asynchronous tester

  // wrapped in an iframe (will not work in SO's StackSnippet®)
  var iframe = document.createElement('iframe');
  iframe.onload = function() {
    var win = iframe.contentWindow;
    // listen for a scroll event
    win.addEventListener('scroll', function handler(e){
      // when the scroll event fires, check that we did move
      if(win.pageXOffset < 99) { // !== 0 should be enough, but better be safe
        attachPolyfill();
      }
      // cleanup
      document.body.removeChild(iframe);      
    });
    // set up our document so we can scroll
    var body = win.document.body;
    body.style.width = body.style.height = '1000px';

    win.scrollTo(10, 0); // force the event
    win.scrollTo({left:100, behavior:'instant'}); // the one we actually test
  };
  // prepare our frame
  iframe.src = "about:blank";
  iframe.setAttribute('width', 1);
  iframe.setAttribute('height', 1);
  iframe.setAttribute('style', 'position:absolute;z-index:-1');
  iframe.onerror = function() {
    console.error('failed to load the frame, try in jsfiddle');
  };
  document.body.appendChild(iframe);

  // The Polyfill

  function attachPolyfill() {
    var original = window.scroll, // keep the original method around
      animating = false, // will keep our timer's id
      dx = 0,
      dy = 0,
      target = null;

    // override our methods
    window.scrollTo = window.scroll = function polyfilledScroll(user_opts) {
      // if we are already smooth scrolling, we need to stop the previous one
      // whatever the current arguments are
      if(animating) {
        clearAnimationFrame(animating);
      }

      // not the object syntax, use the default
      if(arguments.length === 2) {
        return original.apply(this, arguments);
      }
      if(!user_opts || typeof user_opts !== 'object') {
        throw new TypeError("value can't be converted to a dictionnary");
      }

      // create a clone to not mess the passed object
      // and set missing entries
      var opts = {
        left: ('left' in user_opts) ? user_opts.left : window.pageXOffset,
        top:  ('top' in user_opts) ? user_opts.top : window.pageYOffset,
        behavior: ('behavior' in user_opts) ? user_opts.behavior : 'auto',
      };
      if(opts.behavior !== 'instant' && opts.behavior !== 'smooth') {
        // parse 'auto' based on CSS computed value of 'smooth-behavior' property
        // But note that if the browser doesn't support this variant
        // There are good chances it doesn't support the CSS property either...
        opts.behavior = window.getComputedStyle(document.scrollingElement || document.body)
            .getPropertyValue('scroll-behavior') === 'smooth' ?
                'smooth' : 'instant';
      }
      if(opts.behavior === 'instant') {
        // not smooth, just default to the original after parsing the oject
        return original.call(this, opts.left, opts.top);
      }

      // update our direction
      dx = (opts.left - window.pageXOffset) || 0;
      dy = (opts.top - window.pageYOffset) || 0;

      // going nowhere
      if(!dx && !dy) {
        return;
      }
      // save passed arguments
      target = opts;
      // save the rAF id
      animating = anim();

    };
    // the animation loop
    function anim() {
      var freq = 16 / 300, // whole anim duration is approximately 300ms @60fps
        posX, poxY;
      if( // we already reached our goal on this axis ?
        (dx <= 0 && window.pageXOffset <= +target.left) ||
        (dx >= 0 && window.pageXOffset >= +target.left) 
      ){
        posX = +target.left;
      }
      else {
        posX = window.pageXOffset + (dx * freq);
      }

      if(
        (dy <= 0 && window.pageYOffset <= +target.top) ||
        (dy >= 0 && window.pageYOffset >= +target.top) 
      ){
        posY = +target.top;
      }
      else {
        posY = window.pageYOffset + (dx * freq);
      }
      // move to the new position
      original.call(window, posX, posY);
      // while we are not ok on both axis
      if(posX !== +target.left || posY !== +target.top) {
        requestAnimationFrame(anim);
      }
      else {
        animating = false;
      }
    }
  }
})();