iOS Launch Image Loader - meta tags load every image (many large files slow app load/render); this loads only appropriate image

startup-image-loader.js

/*
 * iOS Launch Image Loader    v1.1.1 @ GDibble
 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
 *   Browser downloads all meta-tagged images, thus only append 1 meta tag to
 *   save download time and provide the fastest loading mobile experience.
 *
 * Files must live in '/img/ios/' and named:
 *   startup-2048x1496.png    Landscape    Retina iPads
 *   startup-1536x2008.png    Portrait     Retina iPads
 *   startup-1024x748.png     Landscape    iPad 1 & 2
 *   startup-768x1004.png     Portrait     iPad 1 & 2
 *   startup-1242x2148.png    Portrait     Retina iPhone 6+
 *   startup-750x1294.png     Portrait     Retina iPhone 6
 *   startup-640x1096.png     Portrait     Retina iPhone 5
 *   startup-640x920.png      Portrait     Retina iPhone 4
 *   startup-320x460.png      Portrait     iPhone & iPod Touch
 * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */

(function () {
  function dynamicLaunchImage(prefix, extension) {
    var resolution;
    if (navigator.platform === 'iPad') {
      if (window.devicePixelRatio === 2) {
        //Retina iPads
        if (window.orientation === 90 || window.orientation === -90) {
          resolution = '2048x1496';
        } else {
          resolution = '1536x2008';
        }
      } else {
        //iPad 1 & 2
        if (window.orientation === 90 || window.orientation === -90) {
          resolution = '1024x748';
        } else {
          resolution = '768x1004';
        }
      }
    } else if (window.devicePixelRatio === 3) {
      //Retina iPhone 6+
      resolution = '1242x2148';
    } else if (window.devicePixelRatio === 2) {
      if (window.screen.height === 667) {
        //Retina iPhone 6
        resolution = '750x1294';
      } else if (window.screen.height === 568) {
        //Retina iPhone 5
        resolution = '640x1096';
      } else {
        //Retina iPhone 4
        resolution = '640x920';
      }
    } else {
      //iPhone & iPod Touch
      resolution = '320x460';
    }
    return prefix + resolution + '.' + extension;
  }

  var head    = 'head';
  var headTag = document.getElementsByTagName(head);
  var linkTag = document.createElement('link');
  linkTag.setAttribute('rel', 'apple-touch-startup-image');
  linkTag.href = '/img/ios/' + dynamicLaunchImage('startup-', 'png');
  if (!headTag.length) { //check if head tag exists, create otherwise
    headTag = [ document.createElement(head) ];
    document.body.appendChild(headTag[0]);
  }
  headTag[0].appendChild(linkTag);
})();