Google analytics scroll depth tracking

scrolldepth.js

;(function() {
  var passedMilestones = []
    , nonInteractiveThreshold = 20
  
  window.addEventListener('scroll', function(e) {

    var scrollTop = window.scrollY
      , body = document.body
      , html = document.documentElement
      , docHeight = Math.max(
          body.scrollHeight,
          body.offsetHeight, 
          html.clientHeight,
          html.scrollHeight,
          html.offsetHeight
        )
      , winHeight = window.innerHeight
      , scrollPercent = (scrollTop) / (docHeight - winHeight)
      , scrollPercentRounded = Math.round( scrollPercent * 100 / 10 ) * 10

    if (~passedMilestones.indexOf(scrollPercentRounded)) return
    
    passedMilestones.push(scrollPercentRounded)

    ga(
      'COUNTERNAME.send',
      'event',
      'Scroll depth (' + location.hostname + location.pathname + ')',
      scrollPercentRounded + '%',
      { nonInteraction: scrollPercentRounded < nonInteractiveThreshold }
    )
  })
})()