I suggest reading this posts from Karl Pover:
Have a nice reading !
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Thanks to Pablo and Jahanzeb for the inputs.
Here is what I have done , in sequence, and been able to be in a much better state of understanding of extension objects:
1. Basics of HTML (not too much just basic tags, p, br, table, head, style, body etc)
3. Basics of CSS and Bootstrap
4. Basics of JQuery
But this i have been able to edit/ decode extensions and have created simple extension objects (non selectable yet). w3school is a good resource for all above points. Next is to learn qlik API and then cycling all the points again and again