Javascript asset lazy loading

lazy.js

/**
 * Lazy load assets, call callback fn after all assets have loaded.
 *
 * Requires: jQuery or Zepto
 *
 * Usage:
 * Lazy.load({"js": ["js/file.js"],
 *            "tpl": [{"src": "tpl/event.tpl", "id": "event-view"}],
 *            "css": ["css/file.css"]
 *           });
 *
 * Lazy.load({"js": ["js/file.js"]});
 */
(function() {

window.Lazy = {
    _loaded: {"js":[], "tpl":[], "css":[]},
    load: function(assets, fn) {
        var asset,
            assetsLoaded = 0,
            assetCount = 0,
            onLoad = function(assetType, asset) {
                assetsLoaded++;
                if(asset) Lazy._loaded[assetType].push(asset);
                if(assetsLoaded == assetCount)
                    fn && fn();
            },
            onLoadDelegate = function(assetType, asset) {
                return function() {
                    onLoad(assetType, asset);
                };
            };
    
        for(var type in assets)
            for(var i = 0, al = assets[type].length; i < al; i++)
                assetCount++;
    
        for(var type in assets) {
            switch(type) {
                case "js":
                    for(var i = 0, al = assets[type].length; i < al; i++) {
                        asset = assets[type][i];
                        if(Lazy._loaded[type].indexOf(asset) !== -1) return onLoad();
    
                        var script = document.createElement("script");
                        script.async = true;
                        script.src = asset;
                        script.onload = onLoadDelegate(type, asset);
                        document.body.appendChild(script);
                    }
                break;
    
                case "tpl":
                    for(var i = 0, al = assets[type].length; i < al; i++) {
                        asset = assets[type][i];
                        if(Lazy._loaded[type].indexOf(asset) !== -1) return onLoad();
    
                        var tplEl = $("<script type=\"text/html\" id=\"" + asset.id + "\"></script>");
                        $("body").append(tplEl);
                        tplEl.load(asset.src, onLoadDelegate(type, asset));
                    }
                break;
    
                case "css":
                    for(var i = 0, al = assets[type].length; i < al; i++) {
                        asset = assets[type][i];
                        if(Lazy._loaded[type].indexOf(asset) !== -1) return onLoad();
    
                        var styleEl = $("<style type=\"text/css\"></style>");
                        $("head").append(styleEl);
                        styleEl.load(asset, onLoadDelegate(type, asset));
                    }
                break;
            }
        }
    }
};

})();