Rejection Response Email Discussion

I recently received feedback from a response I wrote after a rejection from a company. I'll paste the relevant parts of the letter without exposing the identity of the company or person kind enough to give me feedback.

Kevin,
...

We found you to have a great breadth of technical domain – it’s clear that your passion for tech has carried you to learn many things, but we found that this breadth may have been achieved at the cost of depth. It’s not only important to know how we can use something, whether a framework, language or library, but by what patterns and how the underlying technology really functions – this is of huge value when things don’t work like they should.

I think you’re on your way to fixing this issue – I found it impressive that you used Node/Express instead of RoR, which I know isn’t what Flatiron taught you. I’d focus in on the best way to use this stack and look at the different patterns that are being seen out there (containerized microservices, MVC designs, Restful principles, etc. This depth will serve you well as a core knowledge, and give you greater context into which you integrate more breadth.

It's after this point that he says goodbye and writes his name which would violate the anonymity granted by the internet so I won't post that information. He was kind enough to give me this input and now I have to do something about it. But what? Any recommendations, be they videos, lectures, books to read, etc. are more than welcome. And any personal experiences or stories about this kind of thing would also be great. I shouldn't be the only person to benefit from this.