Example where data saving will get failed

app.js

var win = Ti.UI.createWindow({
    backgroundColor: '#fff'
});
 
var btn = Ti.UI.createButton({
    title: 'Save File'
});
 
btn.addEventListener('click', function() {
    var key = 'path';
    var dataFile = Ti.Filesystem.getFile(Ti.Filesystem.applicationDataDirectory, key);
    var data = undefined;
    //var data;
    var str = JSON.stringify(data);
    Ti.API.info(str);
    var isFileSaved = dataFile.write(str);
 
    if (isFileSaved) {
        Ti.API.info('file saved');
    } else {
        Ti.API.info('file saving failed');
    }
});
 
win.add(btn);
win.open();