跟踪像素事件滚动长度的Javascript

Question

我想通过设置滚动深度的自定义事件来跟踪用户行为。我真的很喜欢comradsocks.com的设置，在这里，我尝试使用facebook文档将它们作为滚动长度的模型。 https://developers.facebook.com/docs/facebook-pixel/advanced/

这是我遇到的错误。

Uncaught TypeError: Cannot read property 'scrollHeight' of null
    at getDocumentLength ((index):60)
    at getScrollableLength ((index):72)
    at executeWhenReachedPagePercentage ((index):79)
    at (index):117

这是我正在使用的代码。

 var executeWhenReachedPagePercentage = function(percentage, callback) {
  if (typeof percentage !== 'number') {
    console.error(
      'First parameter must be a number, got',
      typeof percentage,
      'instead',
    );
  }

  if (typeof callback !== 'function') {
    console.error(
      'Second parameter must be a function, got',
      typeof callback,
      'instead',
    );
  }

  function getDocumentLength() {
    var D = document;
    return Math.max(
        D.body.scrollHeight, D.documentElement.scrollHeight,
        D.body.offsetHeight, D.documentElement.offsetHeight,
        D.body.clientHeight, D.documentElement.clientHeight
    )
  }

  function getWindowLength() {
    return window.innerHeight || 
      (document.documentElement || document.body).clientHeight;
  }

  function getScrollableLength() {
    if (getDocumentLength() > getWindowLength()) {
      return getDocumentLength() - getWindowLength();
    } else {
      return 0;
    }
  }

  var scrollableLength = getScrollableLength();

  window.addEventListener("resize", function(){
    scrollableLength = getScrollableLength();
  }, false)

  function getCurrentScrolledLengthPosition() {
   return window.pageYOffset || 
     (document.documentElement || document.body.parentNode || document.body).scrollTop;
  }

  function getPercentageScrolled() {
    if (scrollableLength == 0) {
      return 100;
    } else {
      return getCurrentScrolledLengthPosition() / scrollableLength * 100;
    }
  }

  var executeCallback = (function() {
    var wasExecuted = false;
    return function() {
      if (!wasExecuted && getPercentageScrolled() > percentage) {
        wasExecuted = true;
        callback();
      }
    };
  })();

  if (getDocumentLength() == 0 ||
    (getWindowLength()/getDocumentLength() * 100 >= percentage)) {
    callback();
  } else {
    window.addEventListener('scroll', executeCallback, false);
  }
};

    //scroll 50%
executeWhenReachedPagePercentage(50, function() {
  fbq('track', 'SL-ScrollDepth-55');
});

    //scroll 75%
executeWhenReachedPagePercentage(75, function() {
  fbq('track', 'SL-ScrollDepth-75');
});

  //scroll 100%
executeWhenReachedPagePercentage(95, function() {
  fbq('track', 'SL-ScrollDepth-95');
});  




  ///  
</script>

<!-- End Facebook -->

当我滚动时，像素应该触发事件，但是我只是遇到了这个错误。