Web apps these days use frameworks instead of building everything from scratch. The most famous ones being JavaScript frameworks. However there are multiple frameworks available that it becomes confusing to choose one.
In this context here is an one download that has all popular JS frameworks used to create a same application. A todo list which is the Hello world for applications is demonstrated. You can get all the code from the github code repository. These code samples focus on the MV* architecture.

http://addyosmani.github.com/todomvc/