So your extension does not work as expected. Well that happens to all of us. But how do you find the error? Here are some steps that might be a help.
1. Use your favorite browser
The browser that is built in in desktop might cache your extension files, so it is not really suited for extension development. Also in a browser you have better tools for troubleshooting and debugging. I use Google Chrome, since I think the developer tools are pretty good, but other browsers also have developer tools.
2. Look in the console
3. Turn off caching
When you are doing extension development the cache is your enemy: caching might mean that you are actually running an older version of your extension than you think. In Chrome you can turn it off in developer tools, in the network tab:
Note that the cache will only be disabled if you have developer tools open, so you should keep them open when developing.
4. Use the developer tools to look at your source code
Open your source code in the browsers developer tools:
Verify that this is the correct version.
5. Use the debugger to step through your code
The developer tools includes a very good debugger. Use it! Good places to set a breakpoint would be:
any methods triggered by an event (click etc)
Note that you can also see the contents of structures like the layout and evaluate expressions used. And, of course step through your code.
6. Verify that you have only one copy of your extension under Extensions folder
Filenames of your qext file needs to be unique. If you are developing on Qlik Sense desktop it is your own responsibility to make sure that this is the case. If you make backup copies of your extension, do not save them under MyDocuments/Qlik/Sense/Extensions.