// 頁面總高

var totalH = document.body.scrollHeight || document.documentElement.scrollHeight

// 可視高

var clientH = window.innerHeight || document.documentElement.clientHeight

window.addEventListener('scroll', function(e){

    // 計算有效高

    var validH = totalH - clientH

    // 滾動條卷去高度

    var scrollH = document.body.scrollTop || document.documentElement.scrollTop

    // 百分比

    var result = (scrollH/validH*100).toFixed(2)

    console.log(result)

})