Three major methods to achieve image preloading and analysis of their advantages and disadvantages_javascript skills

Preloading images is a great way to improve user experience. With images pre-loaded into the browser, visitors can smoothly surf your site and enjoy blazingly fast loading times. This is very beneficial for image galleries and websites where images take up a large proportion. It ensures that images are published quickly and seamlessly, and it also helps users get a better user experience when browsing your website content. This article will share three different preloading techniques to enhance website performance and usability.

Method 1: Use CSS and JavaScript to implement preloading

There are many ways to implement preloaded images, including using CSS, JavaScript, and various combinations of the two. These technologies can design corresponding solutions according to different design scenarios and are very efficient.
Simply using CSS, you can preload images easily and efficiently. The code is as follows:

Copy code The code is as follows:

#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; }

Applying these three ID selectors to the (X)HTML element, we can preload the image onto the off-screen background through the CSS background attribute. As long as the paths to these images remain the same, the browser will use the preloaded (cached) images during the rendering process when they are called elsewhere on the web page. Simple, efficient and doesn't require any JavaScript.
Although this method is efficient, there is still room for improvement. Images loaded using this method will be loaded together with other content on the page, increasing the overall loading time of the page. To solve this problem, we added some JavaScript code to delay the preloading time until the page is loaded. The code is as follows:

Copy code The code is as follows:

// better image preloading @ http://perishablepress.com/press/2009/12/28/3-ways-preload-images-css-javascript-ajax/">http://perishablepress.com/press/2009/12/28/3-ways-preload-images-css-javascript-ajax/">http: //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() {                                                                                                                                                                                                                                                       window.onload = function()            If (oldonload) {
                        oldonload();                                                                                                                               func();                                                                                                                                  }  
} 
addLoadEvent(preloader);

In the first part of the script, we get the element using the class selector and set the background attribute on it to preload different images.
In the second part of the script, we use the addLoadEvent() function to delay the loading time of the preloader() function until the page is loaded.
What happens if JavaScript doesn't run properly in the user's browser? It's very simple. The image will not be preloaded. When the page calls the image, it will be displayed normally.

Method 2: Use JavaScript only to implement preloading

The above method is sometimes very efficient, but we gradually found that it would consume too much time in the actual implementation process. Instead, I prefer to use pure JavaScript for image preloading. Below are two such preloading methods that work beautifully on all modern browsers.

JavaScript snippet 1
Just simply edit and load the path and name of the required image, it’s easy to implement:

Copy code The code is as follows:

This method is especially suitable for preloading a large number of images. My gallery website uses this technology to preload over 50 images. Apply this script to the login page and most of the gallery images will be preloaded as soon as the user enters their login ID.


JavaScript code snippet 2

This method is similar to the one above and can also preload any number of images. Add the following script to any Web page and edit it according to the program instructions.

Copy code

The code is as follows:

As you can see, each time you load an image, you need to create a variable, such as "img1 = new Image();", and declare the image source address, such as "img3.src = "../path/to/image-003.gif";". Referring to this mode, you can load as many images as you need.

We have further improved this method. Wrap this script into a function and use addLoadEvent() to delay the preloading time until the page is loaded.

Copy code The code is as follows:

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() {                                                                                                                                                                                                                                                       window.onload = function()            If (oldonload) {
                        oldonload();                                                                                                                               func();                                                                                                                                  }  
} 
addLoadEvent(preloader);




Method 3: Use Ajax to implement preloading


The method given above does not seem cool enough, so now let’s look at a method of using Ajax to implement image preloading. This method uses DOM to not only preload images, but also preload CSS, JavaScript and other related things. The advantage of using Ajax over using JavaScript directly is that the loading of JavaScript and CSS will not affect the current page. The method is simple and efficient.

Copy code

The code is as follows:

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('');                                                                             new Image().src = "                                                                               }, 1000);
};



The above code preloads "preload.js", "preload.css" and "preload.png". The 1000 millisecond timeout is to prevent the script from hanging and causing functional issues with normal pages.
Next, let’s see how to use JavaScript to implement the loading process:

Copy code

The code is as follows:

window.onload = function() {       setTimeout(function() {                        // reference to                                                                     var head = document.getElementsByTagName('head')[0];                       // a new CSS                                          var css = document.createElement('link');          css.type = "text/css";                                           css.type = " 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 create three elements through DOM to preload three files. As mentioned above, with Ajax, loading files is not applied to the loading page. From this point of view, Ajax methods are superior to JavaScript.

Okay, this article will first introduce it here. Everyone already knows the three methods of implementing image preloading technology. Which one is more efficient? I think everyone can see it, so apply it to yourself. project.