JSFiddle - React, Tailwind, and code Playground

HTML

<div id="results"></div>

JavaScript

var preloadFile = function(url){
    var start = +new Date();
    var file = document.createElement("img");
    file.src = url;
    return +new Date() - start;
};

var timing = [];
timing.push(preloadFile("http://upload.wikimedia.org/wikipedia/en/f/f7/RickRoll.png"));
timing.push(preloadFile("http://upload.wikimedia.org/wikipedia/en/f/f7/RickRoll.png"));    
    
document.getElementById("results").innerHTML = "First request: " +timing[0]+"<br/>Second request: "+timing[1];

caching = (timing[1] === 0); // Timing[1] should equal zero if caching is enabled