For Hack Reactor it seems that to get in its all about covering and understanding the use of test cases to solve programing questions. This is something that is crucial to the software engineer throughout their career. I have had experience with testing frameworks specially with RSpec during my time at Bloc. Programming is best done with the TDD (Test Driven Development) approach, that is, making unit tests before starting to write actual code. When the tests are ready, then the code writing begins. The goal there is to make all the tests pass. This is a little snapshot of what software engineer everyday routines are.
An update on Fullstack. I interviewed with them and I have been accepted! It was amazing how fast they made their decision. They sent me an acceptance email a day after interviewing. Awesome. Fullstack Academy check!