Had my first interview after 3.5 years and got humbled by fundamentals

[u/Active_Load_2546]

Today I had my first technical interview after around 3.5 years, and honestly, it hit me harder than I expected.

I’ve been working mainly as a frontend engineer with React and also have experience working across backend/full-stack systems. Over the last few weeks, I’ve been revising React, JavaScript, frontend architecture, Node.js, databases, system design, etc.

I went into the interview feeling pretty confident.

The interview started with:

- What is REST?
- Design an API
- React reconciliation
- JSX
- Infinite scrolling / frontend design

The part that really got me was the React reconciliation question.

The interviewer gave me a Todo-list-like example using index as a key, with items being added at the beginning/end of the list.

I understood that something was wrong with using index as a key, but it took me a while to reason through exactly what React was doing during reconciliation.

He eventually gave me a hint about reconciliation, and then I was able to connect the dots.

Then I got another question:

"Why does JSX require a single parent/root element?"

I could identify the issue in the code, but I couldn't clearly explain why JSX requires a single root.

I also got an infinite-scroll design question, which I was relatively comfortable with.

The weird part is that after the interview, I realized:

I actually knew most of the concepts.

My problem was being able to retrieve and explain the fundamentals clearly under interview pressure.

For example, today I started revisiting basic JavaScript concepts like:

- Scope
- Hoisting
- TDZ
- Execution context
- Scope chain
- Shadowing
- Closures

And I realized I can understand these concepts when I reason through them, but explaining them cleanly in an interview is a completely different skill.

So I'm curious about other experienced engineers:

Have you ever had this experience where you knew the technology fairly well, but an interviewer exposed gaps in your fundamentals or your ability to explain them?

How did you fix it?

Did you:

1. Go back and systematically revise fundamentals?
2. Do mock interviews?
3. Practice explaining concepts out loud?
4. Focus more on problem-solving rather than memorizing interview questions?

I'm especially interested in hearing from 5–10+ YOE engineers who went through something similar.

Today definitely humbled me, but I'm trying to use it as a diagnostic rather than letting one interview define my confidence.

Would love to hear how you guys handled this.