This website seems to be aimed at beginners and that’s OK but many from what I have seen seem to give up because they can’t retain the information that they learn or are simply not dedicated enough to learn something so difficult for them to remember. The Pro Quiz feature was a very strong idea to make sure users would retain their information but it doesn’t seem to be enough for some especially those who don’t (or can’t) pay.
Remembering syntax is not that valuable, if programming was about remembering syntax, computers would have taken over developers decades ago, and it still hasn’t happened today
learning a natural language is very different from a programming language, making a comparison between these two can be tricky
many people approach the programming in the wrong way, there are two things people could do generally do better when learning to code on codecademy:
starting at the first lesson, if something is unclear, look at documentation. For JS, MDN has great documentation. Learning to read documentation is vital skill, start with it. Reading documentation also helps if you forget something. Googling will often lead to the right documentation, learning to google the right thing
when completing a track, go fiddle wit the things you learned outside codecademy and build something simple with it. Problem solving is more important then the syntax, for syntax we have documentation.
The cost of developing a similair feature as duolingo has, cost more then it would yield. The feature on duolingo is just repeating what you already did. For programming, this is not as valuable as when learning a natural language
making comments is vital, they tell you what your code does (for yourself for later, and other developers), doing this from the start is a good thing. Learning to write good comments is very important