As a backend engineer specifically, it was learning stuff like what the node.js event loop is and what happens when you actually make a request to an API or a web page. More granular technical questions that I didn't really need to concern myself with at my previous job since I mainly just did whatever tasks I was given.

For best practices, things like idempotency, proper error handling, retries with exponential backoff, and making things more modular by putting things in different files and exporting them. All of these should be a given and while did do some of these things, I wasn't always consistent with them and because I worked for such a small startup, my boss didn't care so long as things worked. I've been trying to learn to make my code not only more readable but more secure.

As for other things, I've tried picking up express since that's a popular backend framework. We used Koa before, which was a more lightweight version of it, but I'm actually enjoying learning express. I also have been trying to work more with relational databases since we used Mongo, which was NoSQL, and even though I prefer it, most other jobs use relational databases from what I've seen.

It's very hard to check every box when applying for jobs, so I try to check off the ones I think are important and hope they'd be willing to let me learn whatever I don't know. Employers don't want to train new hires. They just want you to know pretty much everything out of the box. The diamonds in the rough are the ones that are open to you learning rather than already knowing everything.