I've recently gone through the process of learning how to edit and create extensions. Here is how I learned:
1. Read the first 10 chapters of eloquent javascript. It was enough to get a basic understanding of the the language and DOM. It probably took me about a month to read through this, casually covering some material everyday.
2. Then I read through the w3 schools tutorials on HTML and CSS. This is pretty basic material and didn't take long to cover. Maybe a few hours.
3. Worked through some D3 tutorials. This also took about another month, casually reading through some of the material everyday.
4. Read through the Stefan Walther Qlik Sense extension tutorial. Only takes a few hours
5. Downloaded some extensions on Qlik Branch and started manipulating code.
After covering this material I'm able to edit the look of an extension and add some basic functionality.
