React Hooks useEffect Hook 3

Question 1

What does React do when a dependency in useEffect's array changes?

Accepted Answer

Runs the cleanup of the previous effect, then re-runs the new effect

Answer

React calls the previous effect's cleanup function before re-running the effect with the new dependency values.

Question 2

In React StrictMode (development), how many times does a useEffect fire on mount?

Accepted Answer

Twice (mount, unmount, remount) to detect side-effect issues

Answer

StrictMode intentionally double-invokes effects to help detect missing cleanup logic.

Question 3

What does the eslint-plugin-react-hooks exhaustive-deps rule enforce?

Accepted Answer

All values referenced inside the effect must be listed in the dependency array

Answer

The exhaustive-deps rule warns when you use a value inside useEffect without listing it as a dependency.

Question 4

Which situation would cause an infinite loop with useEffect?

Accepted Answer

Updating state inside the effect that is also a dependency of the same effect

Answer

If you update state that is also listed as a dependency, the effect fires → updates state → fires again endlessly.

Question 5

How should you add an event listener and clean it up with useEffect?

Accepted Answer

Add in useEffect and return a function that removes the listener

Answer

Returning a cleanup function that calls removeEventListener ensures no listener leak when the component unmounts.

React Practice Test