Post-loading : check if an image is in the browser cache

The most efficient, simple, and widely supported way to check if an image has already been cached is to do the following...

1. Create an image object
2. Set the src property to the desired url
3. Check the completed attribute immediately to see if the image is already cached
4. Set the src attribute back to "" (empty string), so that the image is not unnecessarily loaded (unless of coarse you want to load it at this time)

Like so...

function isCached(src) {
    const img = new Image();
    img.src = src;
    const complete = img.complete;
    img.src = "";
    return complete;
}

In your case, it could be implemented like so...

const lazyImages = document.querySelectorAll(".postLoad");
for (const img of lazyImages) {
    if ((!img.src || !isCached(img.src)) && img.getAttribute("alt")) {
        img.src = img.getAttribute("alt");
    }
}

That being said, I'd advise against using the alt attribute for this purpose, you should use something like data-src instead.

An ajax request for the image would return almost immediately if it is cached. Then use setTimeout to determine if its not ready and cancel the request so you can requeue it for later.

Update:

var lqueue = [];
$(function() {
  var t,ac=0;
  (t = $("img")).each(
    function(i,e)
    {
      var rq = $.ajax(
      {
        cache: true,
        type: "GET",
        async:true,
        url:e.alt,
        success: function() { var rq3=rq; if (rq3.readyState==4) { e.src=e.alt; } },
        error: function() { e.src=e.alt; }
      });

      setTimeout(function()
      {
        var k=i,e2=e,r2=rq;
        if (r2.readyState != 4)
        {
          r2.abort();
          lqueue.push(e2);
        }
        if (t.length==(++ac)) loadRequeue();
      }, 0);
    }
  );
});

function loadRequeue()
{
  for(var j = 0; j < lqueue.length; j++) lqueue[j].src=lqueue[j].alt;
}

I have a remark about your empty image sources. You wrote:

So i render the page with empty image sources with the source specified in the "alt" attribute. I insert image sources after the "window.onload" event, and it works great.

I've ran into problems with this in the past, because in some browsers empty src attributes cause extra requests. Here's what they do (copied from Yahoo! performance rules, there's also a blog post on that issue with more detail):

• Internet Explorer makes a request to the directory in which the page is located.
• Safari and Chrome make a request to the actual page itself.
• Firefox 3 and earlier versions behave the same as Safari and Chrome, but version 3.5 addressed this issue[bug 444931] and no longer sends a request.
• Opera does not do anything when an empty image src is encountered.

We also use a lot of jQuery on our site, and it has not always been possible to avoid empty image tags. I've chosen to use a 1x1 px transparent gif like so: src="t.gif" for images that I only insert after pageload. It is very small and gets cached by the browser. This has worked very well for us.

Cheers, Oliver

after some reseach, I found a solution :

The idea is to log the cached images, binding a log function on the images 'load' event. I first thought to store sources in a cookie, but it's not reliable if the cache is cleared without the cookie. Moreover, it adds one more cookie to HTTP requests...

Then i met the magic : window.localStorage (details)

The localStorage attribute provides persistent storage areas for domains

Exactly what i wanted :). This attribute is standardized in HTML5, and it's already works on nearly all recent browsers (FF, Opera, Safari, IE8, Chrome).

Here is the code (without handling window.localStorage non-compatible browsers):

var storage = window.localStorage;
if (!storage.cachedElements) {
    storage.cachedElements = "";
}

function logCache(source) {
    if (storage.cachedElements.indexOf(source, 0) < 0) {
        if (storage.cachedElements != "") 
            storage.cachedElements += ";";
        storage.cachedElements += source;
    }
}

function cached(source) {
    return (storage.cachedElements.indexOf(source, 0) >= 0);
}

var plImages;

//On DOM Ready
$(document).ready(function() {
    plImages = $(".postLoad");

    //log cached images
    plImages.bind('load', function() {
        logCache($(this).attr("src"));
    });

    //display cached images
    plImages.each(function() {
        var source = $(this).attr("alt")
        if (cached(source))
            $(this).attr("src", source);
    });
});

//After page loading
$(window).bind('load', function() {
    //display uncached images
    plImages.each(function() {
        if ($(this).attr("src") == "")
            $(this).attr("src", $(this).attr("alt"));
    });
});