To become a back-end developer is above all to understand that you will spend your days writing codes in JavaScript 😵 (a front-end developer will often in comparison have many other tasks like implementing mockups which will allow him to breathe).
Every day I meet developers who spend their time struggling because they never make the effort to overcome their lack of understanding on features that make up JavaScript or Node.js 😐 (Promise, Symbol, Reflection, Generators, Streams, Event-loop ...).
It is therefore necessary to train and educate yourself much more intensively than your colleagues who aspire to become "mostly" front-end developers (even if the front-end is also becoming more and more technical with sometimes frameworks that require an important mastery of JavaScript).
Interviews are mostly based on your language skills (the more fluent you are, the more likely you are to get the job). Whether it is for an algorithm, a challenge or even the creation of a project (an http API for example).
If you spend ten minutes remembering how to set a key in an object, then you've most likely blown your interview 😭. Having experience with a framework or library is good, but it shouldn't come at the cost of a lack of JavaScript skills.