Images preload via promises

preload.coffee

# helpers
# $ = jQuery

# See: http://learn.jquery.com/code-organization/deferreds/examples/
# Generic asynchronous cache
$.createCache = (requestFunction)->
  cache = {};
  return (key, callback)->
    if ( !cache[key] )
      cache[key] = $.Deferred((defer)->
        requestFunction(defer, key)
      ).promise()
    return cache[key].done(callback)

# load image via defer (promise)
$.loadImage = $.createCache (defer, url)->
  image = new Image()
  cleanUp = -> image.onload = image.onerror = null
  defer.then cleanUp, cleanUp
  image.onload = -> defer.resolve image
  image.onerror = defer.reject
  image.src = url
  
# client code

imagesFiles = ['img1.jpg', 'img2.jpg', 'img3.jpg']

promise = $.when.apply null, imagesFiles.map (url)-> $.loadImage url

promise.fail -> alert 'Something went wrong'
promise.done (img1, img2, img3)-> # do smth

preload.js

// helpers
// $ - jQuery

// See: http://learn.jquery.com/code-organization/deferreds/examples/
// Generic asynchronous cache
$.createCache = function(requestFunction) {
  var cache;
  cache = {};
  return function(key, callback) {
    if (!cache[key]) {
      cache[key] = $.Deferred(function(defer) {
        return requestFunction(defer, key);
      }).promise();
    }
    return cache[key].done(callback);
  };
};

// load image via defer (promise)
$.loadImage = $.createCache(function(defer, url) {
  var cleanUp, image;
  image = new Image();
  cleanUp = function() {
    return image.onload = image.onerror = null;
  };
  defer.then(cleanUp, cleanUp);
  image.onload = function() {
    return defer.resolve(image);
  };
  image.onerror = defer.reject;
  return image.src = url;
});

// client code

var imagesFiles = ['img1.jpg', 'img2.jpg', 'img3.jpg'];

var promise = $.when.apply(null, imagesFiles.map(function(url) {
  return $.loadImage(url);
}));

promise.fail(function() {
  alert('Something went wrong');
});
promise.done(function(img1, img2, img3) {
  // do smth
});