Understanding Javascript image preloading_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 use CSS to preload images easily and efficiently. 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.

While 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:

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 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 code snippet 1

You only need to simply edit and load the path and name of the required image. It is easy to implement:

<div class="hidden">
  <script type="text/javascript">
      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 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 above method and can also preload any number of images. Add the following script to any Web page and edit it according to the program instructions.

<div class="hidden">
  <script type="text/javascript">
      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 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";". Using this pattern, you can load as many images as you need.

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

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);

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.

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);
};

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:

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 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.

Additional information: Loading completion callback function

We write the following code:

function loadImage(url, callback) {
 var img = new Image();
 img.src = url;

 img.onload = function(){ //图片下载完毕时异步调用callback函数。
  callback.call(img);  // 将callback函数this指针切换为img。
 };
}

在firefox中测试一下，发现不错，果然和预想的效果一样，在图片下载后，就会弹出图片的宽度来。无论点击多少次或者刷新结果都一样。

不过，做到这一步，先别高兴太早——还需要考虑一下浏览器的兼容性，于是，赶紧到ie里边测试一下。没错，同样弹出了图片的宽度。但是，再点击load的时候，情况就不一样了，什么反应都没有了。刷新一下，也同样如此。

经过对多个浏览器版本的测试，发现ie6、opera都会这样，而firefox和safari则表现正常。其实，原因也挺简单的，就是因为浏览器的缓存 了。当图片加载过一次以后，如果再有对该图片的请求时，由于浏览器已经缓存住这张图片了，不会再发起一次新的请求，而是直接从缓存中加载过来。对于 firefox和safari，它们视图使这两种加载方式对用户透明，同样会引起图片的onload事件，而ie和opera则忽略了这种同一性，不会引 起图片的onload事件，因此上边的代码在它们里边不能得以实现效果。

怎么办呢？最好的情况是Image可以有一个状态值表明它是否已经载入成功了。从缓存加载的时候，因为不需要等待，这个状态值就直接是表明已经下载了，而从http请求加载时，因为需要等待下载，这个值显示为未完成。这样的话，就可以搞定了。

经过一些分析，终于发现一个为各个浏览器所兼容的Image的属性——complete。所以，在图片onload事件之前先对这个值做一下判断即可。最后，代码变成如下的样子：

function loadImage(url, callback) {
  var img = new Image(); //创建一个Image对象，实现图片的预下载
  img.src = url;
  
  if (img.complete) { // 如果图片已经存在于浏览器缓存，直接调用回调函数
    callback.call(img);
    return; // 直接返回，不用再处理onload事件
  }

  img.onload = function () { //图片下载完毕时异步调用callback函数。
    callback.call(img);//将回调函数的this替换为Image对象
  };
};

虽然代码很简单，但却很实用。 

附：再谈javascript图片预加载

lightbox类效果为了让图片居中显示而使用预加载，需要等待完全加载完毕才能显示，体验不佳（如filick相册的全屏效果）。javascript无法获取img文件头数据，真的是这样吗？本文通过一个巧妙的方法让javascript获取它。

这是大部分人使用预加载获取图片大小的例子：

var imgLoad = function (url, callback) {
  var img = new Image();

  img.src = url;
  if (img.complete) {
    callback(img.width, img.height);
  } else {
    img.onload = function () {
      callback(img.width, img.height);
      img.onload = null;
    };
  };

};
可以看到

上面必须等待图片加载完毕才能获取尺寸，其速度不敢恭维，我们需要改进。

web应用程序区别于桌面应用程序，响应速度才是最好的用户体验。如果想要速度与优雅兼得，那就必须提前获得图片尺寸，如何在图片没有加载完毕就能获取图片尺寸？

十多年的上网经验告诉我：浏览器在加载图片的时候你会看到图片会先占用一块地然后才慢慢加载完毕，并且不需要预设width与height属性，因为浏览器能够获取图片的头部数据。基于此，只需要使用javascript定时侦测图片的尺寸状态便可得知图片尺寸就绪的状态。

当然实际中会有一些兼容陷阱，如width与height检测各个浏览器的不一致，还有webkit new Image()建立的图片会受以处在加载进程中同url图片影响，经过反复测试后的最佳处理方式：

// 更新：
// 05.27: 1、保证回调执行顺序：error > ready > load；2、回调函数this指向img本身
// 04-02: 1、增加图片完全加载后的回调 2、提高性能

/**
 * 图片头数据加载就绪事件 - 更快获取图片尺寸
 * @version  2011.05.27
 * @author  TangBin
 * @see    http://www.planeart.cn/?p=1121
 * @param  {String}  图片路径
 * @param  {Function}  尺寸就绪
 * @param  {Function}  加载完毕 (可选)
 * @param  {Function}  加载错误 (可选)
 * @example imgReady('http://www.google.com.hk/intl/zh-CN/images/logo_cn.png', function () {
    alert('size ready: width=' + this.width + '; height=' + this.height);
  });
 */
var imgReady = (function () {
  var list = [], intervalId = null,

  // 用来执行队列
  tick = function () {
    var i = 0;
    for (; i < list.length; i++) {
      list[i].end &#63; list.splice(i--, 1) : list[i]();
    };
    !list.length && stop();
  },

  // 停止所有定时器队列
  stop = function () {
    clearInterval(intervalId);
    intervalId = null;
  };

  return function (url, ready, load, error) {
    var onready, width, height, newWidth, newHeight,
      img = new Image();
    
    img.src = url;

    // 如果图片被缓存，则直接返回缓存数据
    if (img.complete) {
      ready.call(img);
      load && load.call(img);
      return;
    };
    
    width = img.width;
    height = img.height;
    
    // 加载错误后的事件
    img.onerror = function () {
      error && error.call(img);
      onready.end = true;
      img = img.onload = img.onerror = null;
    };
    
    // 图片尺寸就绪
    onready = function () {
      newWidth = img.width;
      newHeight = img.height;
      if (newWidth !== width || newHeight !== height ||
        // 如果图片已经在其他地方加载可使用面积检测
        newWidth * newHeight > 1024
      ) {
        ready.call(img);
        onready.end = true;
      };
    };
    onready();
    
    // 完全加载完毕的事件
    img.onload = function () {
      // onload在定时器时间差范围内可能比onready快
      // 这里进行检查并保证onready优先执行
      !onready.end && onready();
    
      load && load.call(img);
      
      // IE gif动画会循环执行onload，置空onload即可
      img = img.onload = img.onerror = null;
    };

    // 加入队列中定期执行
    if (!onready.end) {
      list.push(onready);
      // 无论何时只允许出现一个定时器，减少浏览器性能损耗
      if (intervalId === null) intervalId = setInterval(tick, 40);
    };
  };
})();

调用例子：

imgReady('http://www.google.com.hk/intl/zh-CN/images/logo_cn.png', function () {
  alert('size ready: width=' + this.width + '; height=' + this.height);
});

是不是很简单？这样的方式获取摄影级别照片尺寸的速度往往是onload方式的几十多倍，而对于web普通(800×600内)浏览级别的图片能达到秒杀效果。