batches.js

var gCachedAssets = {};

function loadAssets(assetList, callbackFcn) {
	var loadBatch = {
		count: 0,
		total: assetList.length,
		cb: callbackFcn
	};

	for(var i = 0; i < assetList.length; i++) {
		if(gCachedAssets[assetList[i]] == null) {
			var assetType = getAssetTypeFromExtension(assetList[i]);

			if(assetType === 0) { // Asset is an image
				var img = new Image();
				img.onload = function () {
					onLoadedCallback(img, loadBatch);
				};
				img.src = assetList[i];
				gCachedAssets[assetList[i]] = img;

			} else if(assetType === 1) { // Asset is Javascript
				var fileref = document.createElement('script');
				fileref.setAttribute("type", "text/javascript");
				fileref.onload = function (e){
					onLoadedCallback(fileref,loadBatch);
				};
				fileref.setAttribute("src", assetList[i]);
				document.getElementsByTagName("head")[0].appendChild(fileref);
				gCachedAssets[assetList[i]] = fileref;
			}

		} else { // Asset is already loaded
			onLoadedCallback(gCachedAssets[assetList[i]], loadBatch);
		}
	}
}

function onLoadedCallback(asset, batch) {
	batch.count++;
	if(batch.count == batch.total) {
		batch.cb(asset);
	}
}

function getAssetTypeFromExtension(fname) {
	if(fname.indexOf('.jpg') != -1 || fname.indexOf('.jpeg') != -1 || fname.indexOf('.png') != -1 || fname.indexOf('.gif') != -1 || fname.indexOf('.wp') != -1) {
		// It's an image!
		return 0;
	}

	if(fname.indexOf('.js') != -1 || fname.indexOf('.json') != -1) {
		// It's javascript!
		return 1;
	}

	// Uh Oh
	return -1;
}