Simple Script that reads if the mouse is down speed is adjust, jumps back when let go

VideoSpeedListening.js

var myvid = document.getElementsByTagName('video')[0];
    var mouseDown = 0;
document.body.onmousedown = function() { 
    mouseDown = 1;
}
document.body.onmouseup = function() {
    mouseDown = 0;
    myvid.playbackRate = 1;
    myvid.currentTime -=  5;
}
    document.onmousemove = handleMouseMove;
    function handleMouseMove(event) {
        if(mouseDown){
        var height = window.innerHeight || document.documentElement.clientHeight || document.body.clientHeight;
        speed = event.pageY/height;
        console.log(1+(speed))
        myvid.playbackRate = 1+(speed);
        }
    }