JavaScript image preloader with callback

imgpreload.js

/*
 * Copyright (C) 2012 Eike Send
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to
 * deal in the Software without restriction, including without limitation the
 * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
 * sell copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 */

function imgpreload( imgs, callback ) {
  "use strict";
  var loaded = 0;
  var images = [];
  imgs = Object.prototype.toString.apply( imgs ) === '[object Array]' ? imgs : [imgs];
  var inc = function() {
    loaded += 1;
    if ( loaded === imgs.length && callback ) {
      callback( images );
    }
  };
  for ( var i = 0; i < imgs.length; i++ ) {
    images[i] = new Image();
    images[i].onabort = inc;
    images[i].onerror = inc;
    images[i].onload = inc;
    images[i].src = imgs[i];
  }
}

Example usage:

imgpreload(["rainbow.png"], function( images ) {
  canvas.drawImage( images[0], 0, 0 );
});

in inc on line 28, add this.onload = this.onabort = this.onerror = null to clear the handlers. eg. use case: IE6/7 and animated gif, they fire onload on every animation loop.