Sequential images loading with jquery

gistfile1.js

function eachStep(collection, callback, endcallback){
  if(collection.length == 0){
    return endcallback && endcallback();
  }

  jQuery.when(callback(collection[0])).always(function(){
    eachStep(collection.slice(1), callback, endcallback);
  });
}

function loadImage(imageSrc) {
  var deferred = jQuery.Deferred();
  if (typeof loadImageCache[imageSrc] === "undefined") {

    preloader         = new Image();
    preloader.onload  = function() { 
      //console && console.log("Loaded image " + this.src);
      deferred.resolve(this.src) 
    };
    preloader.onerror = function() { 
      console && console.log("Can not load an image " + this.src);
      deferred.reject(this.src)  
    };
    preloader.src     = imageSrc;

    loadImageCache[imageSrc] = true;
  }else{
    console && console.log("Image cached " + imageSrc);
    deferred.resolve(imageSrc);
  }

  return deferred;
};

function getImages(collection, callback){
  eachStep(collection, function(src){
    //console && console.log("Trying to preload image " + src);
    return loadImage(src);
  }, function(){
    console && console.log("Done preloading");
    callback();
  });
}

this code is used in https://github.com/html/jquery-seq