scrollRatio.js

function scrollRatio(percentage) {
    var domHeight = Math.max(
        document.body.scrollHeight,
        document.body.offsetHeight,
        document.documentElement.clientHeight,
        document.documentElement.scrollHeight,
        document.documentElement.offsetHeight);
    return parseInt(window.scrollY / (domHeight - window.innerHeight) * 100 / percentage) * percentage || 0;
}

// jQ 版
function scrollRatio(percentage){
    var winheight = $(window).height();
    var docheight = $(document).height();
    var scrollTop = $(window).scrollTop();
    var trackLength = docheight - winheight;
    return parseInt(scrollTop / trackLength * 100 / percentage) * percentage;
}

// js 版 v2
function getDocHeight() {
    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 amountscrolled(){
    var winheight= window.innerHeight || (document.documentElement || document.body).clientHeight
    var docheight = getDocHeight()
    var scrollTop = window.pageYOffset || (document.documentElement || document.body.parentNode || document.body).scrollTop
    var trackLength = docheight - winheight
    var pctScrolled = Math.floor(scrollTop/trackLength * 100) // gets percentage scrolled (ie: 80 or NaN if tracklength == 0)
    console.log(pctScrolled + '%')
}
 
window.addEventListener("scroll", function(){
    amountscrolled()
}, false)


// js 版 v3
var winheight, docheight, trackLength, throttlescroll
 
function getmeasurements(){
    winheight= window.innerHeight || (document.documentElement || document.body).clientHeight
    docheight = getDocHeight()
    trackLength = docheight - winheight
}
 
function amountscrolled(){
    var scrollTop = window.pageYOffset || (document.documentElement || document.body.parentNode || document.body).scrollTop
    var pctScrolled = Math.floor(scrollTop/trackLength * 100) // gets percentage scrolled (ie: 80 or NaN if tracklength == 0)
    console.log(pctScrolled + '% scrolled')
}
 
getmeasurements()
 
window.addEventListener("resize", function(){
    getmeasurements()
}, false)
 
window.addEventListener("scroll", function(){
    clearTimeout(throttlescroll)
    throttlescroll = setTimeout(function(){ // throttle code inside scroll to once every 50 milliseconds
        amountscrolled()
    }, 50)
}, false)