var speed = document.querySelector('.speed')
var bar=document.querySelector('.speed-bar')
var video=document.querySelector('.flex')

speed.addEventListener('mousemove',function(e){
    e.offsetY
    var mul=e.offsetY /speed.offsetHeight
    var height=Math.round(mul*100)+'%'
    var playbackRate=4*mul
    // var playbackRate=mul*(4-0.4)+0.4
    bar.style.height=height
    bar.textContent=playbackRate.toFixed(2) + 'X'
    video.playbackRate=playbackRate
})