Throughout this book, we learned that debugging is important, especially when we were learning an easier way to figure out the correct ComponentQuery selector. When developing applications with Ext JS, it is mandatory to use a debug tool. Not only for debugging, but you will also be able to learn more about the framework, and it is a great learning exercise.
A few things we always need to remember while creating Ext JS applications (not only Ext JS, but JavaScript in general) are: case sensitive matters, the class LoginScreen is different from Login screen. Be careful with reserved words (http://mattsnider.com/reserved-words-in-javascript/): you cannot use them as namespace, name of classes, and packages nor as variable names. Check spelling, this is very important, sometimes when we are typing, we can type an extra character (fat finger syndrome).
If you were programming in JavaScript almost 10 years ago, our only friend was the dear alert. We used to put several...