filesystem api add content to file

gistfile1.js

// add content to a file
function addContent(existFileName){
  fs.root.getFile(existFileName, {}, function(fileEntry) {
		fileEntry.file(function(file) {
			var reader = new FileReader();
			reader.onloadend = function(e) {
				var sentContent=prompt("Add contents to the file "+existFileName,this.result);
				if (sentContent!=null && sentContent!=""){
					fileEntry.createWriter(function(fileWriter) {
						fileWriter.onwriteend = function(trunc) {
							fileWriter.onwriteend = null; // Avoid an infinite loop.
							var blob = new Blob([sentContent], {type: 'text/plain'});// Create a new Blob and write it to file.
							fileWriter.write(blob);
						}
						fileWriter.onerror = function(e) {
							alert('Sorry, this text could not be saved');
						};
						fileWriter.seek(fileWriter.length); // Start write position at EOF.
  						fileWriter.truncate(0); // truncate existing so all data is written over
					}, errorHandler);
				}
			};
			reader.readAsText(file);
		});
	}, function(e) {
		if (e.code == FileError.NOT_FOUND_ERR){
			alert('Filename does not exists'); // should never happen
		}
	});
}