Get percentage of scroll position

Question

The problem:

What would be the mathematical formula to calculate (regardless of the scrollHeight of the document) how far the bottom of the scrollbar i

Answer 1

Returns a number between 0 to 100 relative to scroll position:

document.onscroll = function(){ 
  var pos = getVerticalScrollPercentage(document.body)
  document.body.innerHTML = "" + Math.round(pos) + "%"
}

function getVerticalScrollPercentage( elm ){
  var p = elm.parentNode
  return (elm.scrollTop || p.scrollTop) / (p.scrollHeight - p.clientHeight ) * 100
}

body{ height:2000px }
span{ position:fixed; font:5em Arial; color:salmon; }

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● Difference between scrollHeight & clientHeight