JS custom caching function?

Question

I'd like to initially load a "Caching" function for my Chrome app, then run the actual code, like so:

function preloader(){
    hideDOM();
    loadSpinner();
    for(var i=0;i<x;i++)
        caching (loading resources)...
}

function startAPP(){
    showDOM();
    stopSpinner();
    loadAPP();
}

So what I need is the preloader first, then when the "for" loop ends, load the startAPP.

What does the caching step do exactly? Does it load resources? If so, a synchronous loop isn't going to work. Attach onload events to the resources so you'll know when you're done loading. — Halcyon
– Halcyon, Commented Sep 1, 2015 at 15:59
@Halcyon Yes, the caching loads some files to disable latency and delay issues. Could you give me an example for attaching the onload to events please? — Niko
– Niko, Commented Sep 1, 2015 at 16:03

Halcyon · Accepted Answer · 2015-09-01 16:08:33Z

Here is a proof of concept that I cobbled together: (there might be bugs)

function preload_resources(callback) {
    var images = [
        "http://domain/img1.jpg",
        "http://domain/img2.jpg",
        "http://domain/img3.jpg"
    ];

    // count the number of completed load events
    var count = 0;
    function done() {
        count += 1;
        if (count === images.length) {  // all images loaded
            callback();
        }
    }

    // make a hidden div;
    var div = document.createElement("div");
    div.style.display = "none";
    document.body.appendChild(div);

    // add images to DOM
    images.forEach(function (image_url) {
        var img = document.createElement("img");
        img.src = image_url;
        // attach load event
        img.addEventListener("load", done);
        div.appendChild(img);
    });
}

hideDOM();
loadSpinner();
preload_resources(function () {
    showDOM();
    stopSpinner();
    loadAPP();
});

It loads images, in theory you can load anything you want.

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