What Separates a Memorized Answer From Real React Developer Skill

Memorized Answer:
"useEffect runs after every render. You pass a
dependency array to control when it runs. If
you pass an empty array, it runs once."

Why it sounds memorized:
- It is correct.
- It is also what 100% of candidates say.
- It does not explain why it exists, what it is
  for, or what happens when it goes wrong.

Result: the interviewer asks a follow-up and the
candidate's reasoning collapses.

Real Skill Answer:
"useEffect is React's way of letting a component
synchronise with something outside of React —
a network request, a subscription, a DOM API,
a timer.

It runs after the commit phase, so the browser
has already painted. That is why it is safe to
measure the DOM in an effect but not during
render.

The dependency array decides when it re-runs.
But the part that matters in production is the
cleanup function. If you set up a subscription
and do not tear it down, you get memory leaks,
double handlers in strict mode, and stale
closures.

I have seen that bug more times than I can
count in code review."

Why it sounds real:
- It explains the purpose, not just the API.
- It names the timing and the reason.
- It names the bug the person has actually
  fixed.

Memorised phrase:
"Context is for global state."

Why it fails the follow-up:
- Every consumer re-renders when the value
  changes, even if it uses only one field.
- That is the trade-off. If you do not know
  it, the follow-up exposes you.

Real Skill phrase:
"Context avoids prop drilling, but it is not a
state manager. Every consumer re-renders when
the value object changes.

That is why we split context by concern and
memoise the provider value — otherwise a
change in the cart count re-renders the
entire app."

Why it works:
- It names the benefit.
- It names the cost.
- It names the fix.

Weak:
Interviewer: "Why is the empty array there?"
Candidate: "Because it runs once."
Interviewer: "Why does it run once?"
Candidate: "Because that is what the docs say."

Result: the conversation stops. The candidate
memorised the rule, not the reason.

Strong:
Interviewer: "Why is the empty array there?"
Candidate: "Because this effect has no
dependencies on props or state — it is a
one-time subscription. If I added a prop
here, I would need to add it to the array,
otherwise the effect would capture a stale
value."

Interviewer: "What happens in strict mode?"
Candidate: "It runs twice in development, so
the cleanup has to actually unsubscribe —
otherwise you get two listeners and
double-fetching."

Result: the interviewer stops testing and
starts discussing.

Phrases that sound memorised:
- "setState is async."
- "Keys help React identify items."
- "Context is for global state."
- "useMemo memoises a value."
- "Hooks are functions that let you use state."

Every one of these is technically correct.
Every one of these fails the follow-up.

Replacements:
- "Updates are batched; state is a snapshot
  within a render."
- "Keys must be stable, or React reuses the
  wrong DOM node."
- "Context removes prop drilling, but every
  consumer re-renders on change."
- "useMemo is for expensive calculations —
  not a default optimisation."
- "Hooks let components share logic without
  class components or HOCs."

Why it matters:
- The memorised phrase sounds correct but
  shallow.
- The replacement sounds like someone who
  has debugged the problem.
- The follow-up is where the difference is
  decided.