I would say that Hacker Rank helps with learning syntax of a language. I'm not sure if it helps greatly with learning how to implement framework type ideas or large programs.

Hacker Rank is best at teaching algorithms & problem solving. I did a couple the other day & at first I thought they looked difficult. After breaking them down & thinking about them, they surprisingly weren't all that hard.

The approach is actually learn enough about the language (through docs or a book) and practice such knowledge solving code challenges and writing your on tests first to get exposed for a more real environment.

Last time I tried hackerrank i wasn't impressed - the problem specification was lacking, input/output wasn't defined well enough. Quality varied wildly between problems.

I'm mildly familiar with both. I'm of the opinion that hackerrank would be better for learning a language, whereas Euler might be better for math problem solving.