JSFiddle - React, Tailwind, and code Playground

JavaScript

alert("Click \"OK\" to start the test");
var imgAddr = "http://upload.wikimedia.org/wikipedia/commons/2/2d/Snake_River_%285mb%29.jpg" + "?n=" + Math.random();
var startTime, endTime;
var download_size = 5*1024*1024;
var img = new Image();
img.onload = function () {
    endTime = (new Date()).getTime();
    ShowData();
}
startTime = (new Date()).getTime();
img.src = imgAddr;

function ShowData()
{
    var duration = (endTime - startTime) / 1000;
    var bitsLoaded = download_size * 8;
    var speedMbps = ((bitsLoaded / duration) / 1024 / 1024).toFixed(2);
    alert("Speed: " + speedMbps + " Mbps");
}