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

To manage component state

Answer

To memoize expensive computations

Answer

To create context values

Question 2

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

Accepted Answer

Only once after the initial render

Answer

On every render

Answer

Only when dependencies change

Answer

Never

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

Prevents the component from re-rendering

Answer

Clears the state

Answer

Triggers a new render

Question 4

What happens when you omit the dependency array from useEffect?

Accepted Answer

The effect runs after every render

Answer

The effect runs only once

Answer

The effect never runs

Answer

React throws an error

Question 5

Which of the following is a valid useEffect cleanup?

Accepted Answer

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

Answer

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

Answer

useEffect(() => clearInterval(id))

Answer

useEffect.cleanup(() => clearInterval(id))

Question 6

In which phase does useEffect run relative to the browser paint?

Accepted Answer

After the browser has painted

Answer

Before the DOM is updated

Answer

Before the browser paints

Answer

Synchronously during render