filesystem.js

/**
 *  Prepare the App Folder 
 */
(function(){
	window.appRootDirName = ".myapp";
	document.addEventListener("deviceready", onDeviceReady, false);

	function onDeviceReady() {
		console.log("device is ready");
		window.requestFileSystem  = window.requestFileSystem || window.webkitRequestFileSystem;
		window.requestFileSystem(LocalFileSystem.PERSISTENT, 0, gotFS, fail);
	}
	
	function fail() {
		console.log("failed to get filesystem");
	}

	function gotFS(fileSystem) {
		console.log("filesystem got");
		fileSystem.root.getDirectory(window.appRootDirName, {
			create : true,
			exclusive : false
		}, dirReady, fail);
	}

	function dirReady(entry) {
		window.appRootDir = entry;
		console.log(JSON.stringify(window.appRootDir));
	}
})();

imageutil.js

function downloadImage(url, success, err){
	console.log('download image ' + url);
	
	var fileName = new Date().getTime() + ".png";
	ft = new FileTransfer();
	ft.download(
	    url,
	    window.appRootDir.fullPath + "/" + fileName,
	    function(entry) {
	        console.log("download complete: " + entry.fullPath);
	        success(entry.fullPath);
	    },
	    function(error) {
	        console.log("download error source " + error.source);
	        console.log("download error target " + error.target);
	        console.log("upload error code" + error.code);
	        err(error);
	    }
	);
}