Bookmarklet to change aspect ratio of html5 video

stretchvideo.js

document.dispatchEvent(new Event('mobi.omegacentauri.killStretchVideoEvent'));
var vid = document.getElementsByTagName('video');
if (vid.length == 0) {
    alert("No video elements found in this page.");
}
else {
    var mode = prompt("Enter one of these:\n"+
                      "(a) horizontal,vertical : a pair of scale ratios\n"+
                      "(b) horizontal:vertical : the correct aspect ratio for a badly encoded video\n", "1,1");
    
    var stretchX = null;
    var stretchY = null;
    var ratio = null;
    
    if (mode != null) {
        var colon = mode.indexOf(":");
        if (colon >= 0) {
            ratio = parseFloat(mode.substring(0,colon).trim()) / parseFloat(mode.substring(colon+1).trim());
        }
        else {
            var comma = mode.indexOf(",");
            if (comma < 0) {
                stretchX = parseFloat(mode);
                stretchY = 1;
            }
            else {
                stretchX = parseFloat(mode.substring(0,comma).trim());
                stretchY = parseFloat(mode.substring(comma+1).trim());                
            }
        }
    
        function stretchVideo(v) {
            if (v.style.background != "black")
                v.style.background = "black";
            var transform = null;
            if (stretchX != null) {
                transform = "scale("+stretchX+","+stretchY+")";
            }
            else if (ratio > 0) {
                if (v.style.objectFit != "contain")
                    v.style.objectFit = "contain";
                videoRatio = v.videoWidth / v.videoHeight;
                if (videoRatio >= ratio) {
                    transform = "scaleX("+(ratio/videoRatio)+")";
                }
                else {
                    transform = "scaleY("+(videoRatio/ratio)+")";
                }
            }
            if (transform != null && v.style.webkitTransform != transform)
                v.style.webkitTransform=transform;
        }

        function stretchAll() {
            var vid = document.getElementsByTagName('video');
            for(var i=0;i<vid.length;i++)
                stretchVideo(vid[i]);
        }

        stretchAll();

        function resizeListener() {
            setTimeout(stretchAll, 1000);
        }
        if (typeof(ResizeObserver) != "undefined") {
            var observer = new ResizeObserver(function(entries) {
                for (let e of entries) {
                    setTimeout(function() { stretchVideo(e.target) }, 1000);
                }
            });        
            for (var i=0;i<vid.length;i++)
                observer.observe(vid[i]);
        }
        else {
            observer = null;
        }
        function killListener() {
            document.removeEventListener('mobi.omegacentauri.killStretchVideoEvent', killListener);
            window.removeEventListener('resize', resizeListener);
            if (observer != null) 
                for(var i=0;i<vid.length;i++)
                    observer.unobserve(vid[i]);
        }
        document.addEventListener('mobi.omegacentauri.killStretchVideoEvent', killListener);           
    }
}