Most developers know how to use React — few truly understand how it works under the hood.
Out of hundreds of interviews I’ve cleared, failed, or taken as an interviewer, there’s one question that challenged me more than any other:
“Explain how React actually works under the hood.”
I went in confident.
I’ve built 24+ production React applications.
I use hooks daily.
I optimize renders.
I’ve shipped features under impossible deadlines.
But the moment I began answering, I realized something embarrassing:
I was only describing how to use React, not how it works.
“Uh… you use _useState_ for state… _useEffect_ for side effects… components return JSX…”
The interviewer smiled.
“That’s how you use React. Seriously?”
That moment humbled me.
And it taught me the gap between React API knowledge and React fundamentals.
How React Actually Works — What I Wish I Knew Then
1. Reconciliation: The Real Workhorse
React doesn’t magically update the UI.
It performs a process called reconciliation, where:
- It creates a new Virtual DOM tree.
- Compares it with the previous one (diffing).
- Updates only the parts of the real DOM that changed.
This is why React apps feel fast and efficient.
2. React Fiber: The Secret Engine
React Fiber is a complete rewrite of React’s core algorithm.
Get Richa Gautam 🌷’s stories in your inbox
Join Medium for free to get updates from this writer.
Subscribe
Subscribe
It allows React to:
- Break rendering into small units of work
- Prioritize important updates (like user input)
- Pause or resume work without blocking the UI
- Keep the UI responsive during heavy tasks
Fiber is why React apps don’t freeze when things get complex.
3. Concurrent Rendering: React Thinking Ahead
Modern React is concurrent.
This means React can:
- Prepare UI updates in the background
- Throw away outdated renders
- Keep the screen showing the most relevant state
- Avoid janky frames during heavy updates
It’s the backbone of features like _useTransition_ and Suspense.
The Follow-Up Question That Revealed Everything
The interviewer then asked:
“So… when would understanding this matter in your daily work?”
This is where most developers fail — not because they don’t know React APIs, but because they lack fundamental intuition.
Here’s what interviewers actually care about:
Why Understanding React Internals Matters
✔ Debugging unnecessary re-renders
✔ Knowing why state updates don’t apply instantly
✔ Deciding when to use useMemo, useCallback, or React.memo
✔ Solving performance bottlenecks like a senior engineer
✔ Building smooth UIs under heavy load
This is the difference between:
❌ Someone who uses React
✔ Someone who understands React
I Didn’t Clear That Round — But It Transformed Me
I didn’t pass that interview.
But that one question completely changed how I write React, debug React, and teach React.
Today, I ask candidates the same question — not to reject them, but to understand how deep their fundamentals truly go.
Because knowing React APIs makes you a developer.
Understanding React internals makes you a great one.
🙏 Stay Connected!
🔔 Follow for more guides on JavaScript, React.js & interview prep.
**_Thankyou! for reading my article.
Top comments (0)