Browsers load images after an HTTP request is sent for them either by <img> tag or through a javascript call. If you have a javascript function that changes an image on mouseover, you have to wait while the image is retrieved from the server. This would be disturbing when users have a slow internet connection or when the images being retrieved are very large. Although most of the browsers store images in the local cache so that subsequent calls to them are responded immediately, there is still a delay in the first time the images are needed. 

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For better display and visitor interaction, image preloading must be implemented in some cases.  Previously we focused on two techniques to achieve image preloading in the following posts:

• Image Preloading with Javascript
• Preloading Images Using CSS

Today, I would like to compare these two methods to understand the safest and easiest method for preloading images:

Preloading with Javascript:

In order to preload images with Javascript, browsers should support Javascript and it should be turned on. Without Javascript, the preloading will not happen and each time the image is needed, it will be fetched. If the preloaded images will be used on image effects such as rollovers or other effects that require Javascript, preloading with Javascript is the best choice. That way, for browsers that are not supporting Javascript, the images will not be preloaded. 

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Preloading images decreases page load time and enhance user experience. In the previous articles: Image Preloading with Javascript and Image Preloading with jQuery, we focused on techniques to preload images using Javascript.

In this post, I would like to focus on another image preload technique using CSS. And on another post, I compared two image preloading techniques to understand advantages and disadvantages of both method - Preloading Images: Javascript or CSS – Comparing the techniques

Image preloading with CSS can be achieved by a simple <div> with style “display:none;” containing all <img> tags of images to be preloaded. 

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Image Preloading is a technique, often used in navigation graphics and image rollovers, to load images and put them in the browser's cache before they are needed. In this post, we will focus on the Javascript technique to implement image preloading. Also, check another Javascript image preloading technique on Image Preloading with jQuery

The Javascript Image() Object

The simplest way to preload an image is to instantiate a new Image() object in Javascript and pass the URL of the image you want to preload.

For example, if you want to preload an image, simply create a new javascript Image() object and place the script in the head section of the page to ensure it runs as soon as the page loads.

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Pre-loading images is the key point to prevent the flickering while rendering a cool JavaScript driven animation, rollover, Ajax loader or toggle effect. If the script which is replacing an image pauses to wait for another image to load, the effect became useless. Image preloading which is pretty simple with jQuery is the solution:

PRELOADING ARRAY OF IMAGES:

The code snippet that is around the Internet for some time provides image preloading:

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