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Summary of js preloading image methods_javascript skills

May 16, 2016 pm 03:55 PM

js picture Preloading

The examples in this article summarize the js preloading image method. Share it with everyone for your reference. The specific analysis is as follows:

1. Pure 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;
}

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2. JS CSS optimization:

// better image preloading @ 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() {
      if (oldonload) {
        oldonload();
      }
      func();
    }
  }
}
addLoadEvent(preloader);

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3. JS code 1:

&lt;div class="hidden"&gt;
  &lt;script type="text/javascript"&gt;
    &lt;!--//--&gt;&lt;![CDATA[//&gt;&lt;!--
      var images = new Array()
      function preload() {
        for (i = 0; i &lt; 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"
      )
    //--&gt;&lt;!]]&gt;
  &lt;/script&gt;
&lt;/div&gt;

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4. JS code 2:

&lt;div class="hidden"&gt;
  &lt;script type="text/javascript"&gt;
    &lt;!--//--&gt;&lt;![CDATA[//&gt;&lt;!--
      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";
      }
    //--&gt;&lt;!]]&gt;
  &lt;/script&gt;
&lt;/div&gt;

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5. JS code optimization 2:

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);

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6. Ajax code 1:

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);
};

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7. Ajax code 2:

window.onload = function() {
  setTimeout(function() {
    // reference to &lt;head&gt;
    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);
};

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I hope this article will be helpful to everyone’s JavaScript programming design.

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