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3 Ways to Preload Images with CSS, JavaScript, or Ajax

Preloading images is a great way to improve the user experience. When images are preloaded in the browser, the visitor can surf around your site and enjoy extremely faster loading times. This is especially beneficial for photo galleries and other image-heavy sites where you want to deliver the goods as quickly and seamlessly as possible. Preloading images definitely helps users without broadband enjoy a better experience when viewing your content. In this article, we’ll explore three different preloading techniques to enhance the performance and usability of your site.

Method 1: Preloading with CSS and JavaScript

There are many ways to preload images, including methods that rely on CSS, JavaScript, and various combinations thereof. As one of my favorite topics here at Perishable Press, I have covered image preloading numerous times:

Each of these techniques sort of builds on previous methods and remains quite effective and suitable for a variety of design scenarios. Thankfully, readers always seem to chime in on these posts with suggestions and improvements. Recently, Ian Dunn posted an article (404 link removed 2013/08/21) that improves upon my Better Image Preloading without JavaScript method.

With that method, images are easily and effectively preloaded using the following CSS:

#preload-01 { background: url(http://domain.tld/image-01.png) no-repeat -9999px -9999px; }
#preload-02 { background: url(http://domain.tld/image-02.png) no-repeat -9999px -9999px; }
#preload-03 { background: url(http://domain.tld/image-03.png) no-repeat -9999px -9999px; }

By strategically applying preload IDs to existing (X)HTML elements, we can use CSSbackground property to preload select images off-screen in the background. Then, as long as the paths to these images remains the same when they are referred to elsewhere in the web page, the browser will use the preloaded/cached images when rendering the page. Simple, effective, and no JavaScript required.

As effective as this method is, however, there is room for improvement. As Ian points out, images that are preloaded using this method will be loaded along with the other page contents, thereby increasing overall loading time for the page. To resolve this issue, we can use a little bit of JavaScript to delay the preloading until after the page has finished loading. This is easily accomplished by applying the CSS background properties using Simon Willison’s addLoadEvent() (404 link removed 2012/10/18) script:

// better image preloading @ https://perishablepress.com/press/2009/12/28/3-ways-preload-images-css-javascript-ajax/
function preloader() {
	if (document.getElementById) {
		document.getElementById("preload-01").style.background = "url(http://domain.tld/image-01.png) no-repeat -9999px -9999px";
		document.getElementById("preload-02").style.background = "url(http://domain.tld/image-02.png) no-repeat -9999px -9999px";
		document.getElementById("preload-03").style.background = "url(http://domain.tld/image-03.png) no-repeat -9999px -9999px";
	}
}
function addLoadEvent(func) {
	var oldonload = window.onload;
	if (typeof window.onload != 'function') {
		window.onload = func;
	} else {
		window.onload = function() {
			if (oldonload) {
				oldonload();
			}
			func();
		}
	}
}
addLoadEvent(preloader);

In the first part of this script, we are setting up the actual preloading by targeting specific preload elements with background styles that call the various images. Thus, to use this method, you will need to replace the “preload-01”, “preload-02”, “preload-03”, etc., with the IDs that you will be targeting in your markup. Also, for each of the background properties, you will need to replace the “image-01.png”, “image-02.png”, “image-03.png”, etc., with the path and name of your image files. No other editing is required for this technique to work.

Then, in the second part of the script, we are using the addLoadEvent() function to delay execution of our preloader() function until after the page has loaded.

SO what happens when JavaScript is not available on the user’s browser? Quite simply, the images will not be preloaded and will load as normal when called in the web page. This is exactly the sort of unobtrusive, gracefully degrading JavaScript that we really like :)

Method 2: Preloading with JavaScript Only

As effective as the previous method happens to be, I generally find it to be too tedious and time-consuming to actually implement. Instead, I generally prefer to preload images using a straight-up slice of JavaScript. Here are a couple of JavaScript-only preloading methods that work beautifully in virtually every modern browser..

JavaScript Method #1

Unobtrusive, gracefully degrading, and easy to implement, simply edit/add the image paths/names as needed — no other editing required:

<div class="hidden">
	<script type="text/javascript">
		<!--//--><![CDATA[//><!--
			var images = new Array()
			function preload() {
				for (i = 0; i < preload.arguments.length; i++) {
					images[i] = new Image()
					images[i].src = preload.arguments[i]
				}
			}
			preload(
				"http://domain.tld/gallery/image-001.jpg",
				"http://domain.tld/gallery/image-002.jpg",
				"http://domain.tld/gallery/image-003.jpg"
			)
		//--><!]]>
	</script>
</div>

This method is especially convenient for preloading large numbers of images. On one of my gallery sites, I use this technique to preload almost 50 images. By including this script on the login page as well as internal money pages, most of the gallery images are preloaded by the time the user enters their login credentials. Nice.

JavaScript Method #2

Here’s another similar method that uses unobtrusive JavaScript to preload any number of images. Simply include the following script into any of your web pages and edit according to the proceeding instructions:

<div class="hidden">
	<script type="text/javascript">
		<!--//--><![CDATA[//><!--

			if (document.images) {
				img1 = new Image();
				img2 = new Image();
				img3 = new Image();

				img1.src = "http://domain.tld/path/to/image-001.gif";
				img2.src = "http://domain.tld/path/to/image-002.gif";
				img3.src = "http://domain.tld/path/to/image-003.gif";
			}

		//--><!]]>
	</script>
</div>

As you can see, each preloaded image requires a variable definition, “img1 = new Image();”, as well as a source declaration, “img3.src = "../path/to/image-003.gif";”. By replicating the pattern, you can preload as many images as necessary. Hopefully this is clear — if not, please leave a comment and someone will try to help you out.

We can even improve this method a bit by delaying preloading until after the page loads. To do this, we simply wrap the script in a function and use addLoadEvent() to make it work:

function preloader() {
	if (document.images) {
		var img1 = new Image();
		var img2 = new Image();
		var img3 = new Image();

		img1.src = "http://domain.tld/path/to/image-001.gif";
		img2.src = "http://domain.tld/path/to/image-002.gif";
		img3.src = "http://domain.tld/path/to/image-003.gif";
	}
}
function addLoadEvent(func) {
	var oldonload = window.onload;
	if (typeof window.onload != 'function') {
		window.onload = func;
	} else {
		window.onload = function() {
			if (oldonload) {
				oldonload();
			}
			func();
		}
	}
}
addLoadEvent(preloader);

Ahhh, the joys of JavaScript!

Method 3: Preloading with Ajax

As if all of that weren’t cool enough, here is a way to preload images using Ajax. This method was discovered at Of Geeks and letters, and uses the DOM to preload not only images, but CSS, JavaScript, and just about anything else. The main benefit of using Ajax over straight JavaScript is that JavaScript and CSS files can be preloaded without their contents affecting the current page. For images this is not really an issue, but the method is clean and effective nonetheless.

window.onload = function() {
	setTimeout(function() {
		// XHR to request a JS and a CSS
		var xhr = new XMLHttpRequest();
		xhr.open('GET', 'http://domain.tld/preload.js');
		xhr.send('');
		xhr = new XMLHttpRequest();
		xhr.open('GET', 'http://domain.tld/preload.css');
		xhr.send('');
		// preload image
		new Image().src = "http://domain.tld/preload.png";
	}, 1000);
};

As is, this code will preload three files: “preload.js”, “preload.css”, and “preload.png”. A timeout of 1000ms is also set to prevent the script from hanging and causing issues with normal page functionality.

To wrap things up, let’s look at how this preloading session would look written in plain JavaScript:

window.onload = function() {

	setTimeout(function() {

		// reference to <head>
		var head = document.getElementsByTagName('head')[0];

		// a new CSS
		var css = document.createElement('link');
		css.type = "text/css";
		css.rel  = "stylesheet";
		css.href = "http://domain.tld/preload.css";

		// a new JS
		var js  = document.createElement("script");
		js.type = "text/javascript";
		js.src  = "http://domain.tld/preload.js";

		// preload JS and CSS
		head.appendChild(css);
		head.appendChild(js);

		// preload image
		new Image().src = "http://domain.tld/preload.png";

	}, 1000);

};

Here we are preloading our three files upon page load by creating three elements via the DOM. As mentioned in the original article, this method is inferior to the Ajax method in cases where the preloaded file contents should not be applied to the loading page.

Know some triks?

I love these preloading tricks so much, I could just squeeze something. If you know of any good preloading tricks, including any improvements to the techniques shared here, kick start my heart with your wise words of preloading wisdom ;)

Jeff Starr
About the Author Jeff Starr = Web Developer. Security Specialist. WordPress Buff.
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41 responses
  1. Great tips Jeff, thanks a lot. For you what’s the best method? :)

  2. Montana Flynn December 28, 2009 @ 10:20 am

    Great article Jeff, I will be using this for my new projects. Thanks. PS. Can you write an article about caching images?

  3. Jeff Starr

    I tend to prefer JavaScript Method #1, mostly because of its easy implementation.

  4. This is a very good post and i also like your blog page layout too. Bookmarked your site and will stop by again

  5. Miller Medeiros December 28, 2009 @ 12:47 pm

    another technique that’s even simpler is to add the images that you need to preload inside a ‘div’ with display ‘none’ inside the HTML… that way you don’t need JavaScript or any extra CSS to preload the images.

    configure Etags and add an expires header to the images.. that way you will force a strong caching and will make the preload work better.

    I’m not sure if it happen this way on all browsers but Firefox loads inline images before CSS background images. So using this technique can be useful if for instance you need to load the “css sprites” before the actual content images (I’m using this technique on my website..) – If you want to check the load order on any website just enable the “net panel” on Firebug to test it.

    cheers.

  6. Jeff Starr

    @Montana Flynn: Actually yes, I have something in mind for a caching article and will be working on it this coming month. Stay tuned..

    @JohnKraft: Thanks for the feedback :)

    @Miller Medeiros: Absolutely – that is another fine way to preload images. At the beginning of this article, I link to my initial post using the CSS-only method, and also the follow-up article, wherein I attempt to improve the original technique.

  7. I’ve used:

    for preloading images/URLs for Firefox browsers for ages :D I think there are some other browsers that support it too.

    A nice simple way (place code in the head of html doc).

  8. Jeff Starr

    @Deano: It looks like WordPress ate your code – could you try again or perhaps send the code to me at “jeff” at this domain and I will post manually..? Thanks.

  9. Alex Flueras December 28, 2009 @ 10:22 pm

    The code is not eaten :). You just have to place your pointer over the code and it will expand… Great article, thanks for sharing.

  10. Nice article!

    Css sprites:
    I saw a very cool sprite conversiom at this site: http://www.makebetterwebsites.com/

    David uses one very small image for the entire page and in may eyes it works great.

    What do you think about it?? :-)

  11. Jeff Starr

    Hi justin, image sprites are a great way to improve performance by reducing the number of HTTP requests. I use them here at Perishable Press and on many other sites — they are an important tool and a good way to preload your CSS background images. The preloading method described in this article is actually geared more toward caching of bulk images such as used in galleries, portfolios, shopping carts, etc., but also works fine for preloading sprites.

  12. Incase you cant read this again – your image sources on the second method are repeated rather than incrementing the image number…
    copy and paste are a bitch.

    img1.src = "http://domain.tld/path/to/image-001.gif";
    img1.src = "http://domain.tld/path/to/image-002.gif";
    img1.src = "http://domain.tld/path/to/image-003.gif";

    you mean:

    img1.src = "http://domain.tld/path/to/image-001.gif";
    img2.src = "http://domain.tld/path/to/image-002.gif";
    img3.src = "http://domain.tld/path/to/image-003.gif";

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