React Hooks useEffect Hook

Question 1

What is the primary purpose of the useEffect hook?

Accepted Answer

To perform side effects in function components

Answer

useEffect lets you synchronize a component with external systems by running side effects after render.

Question 2

When does a useEffect with an empty dependency array [] run?

Accepted Answer

Only once after the initial render

Answer

An empty dependency array tells React to run the effect only once, after the first render.

Question 3

What does the cleanup function returned from useEffect do?

Accepted Answer

Runs before the next effect or on unmount to cancel subscriptions

Answer

The cleanup function runs before the effect re-fires and on component unmount to avoid memory leaks.

Question 4

What happens when you omit the dependency array from useEffect?

Accepted Answer

The effect runs after every render

Answer

Without a dependency array, useEffect runs after every completed render.

Question 5

Which of the following is a valid useEffect cleanup?

Accepted Answer

useEffect(() => { return () => clearInterval(id); })

Answer

The cleanup must be a function returned from the effect callback, not the direct result of clearInterval.

React Practice Test