Side Effects with useEffect Quiz

Question 1

When does the body of a useEffect run?

Accepted Answer

After React has committed the render and the browser has painted

Answer

During rendering, before the DOM is updated

Answer

Only once when the app starts

Answer

Before the component's first render

Question 2

What does an empty dependency array mean?

Accepted Answer

The effect runs once after the first render, and its cleanup runs on unmount

Answer

The effect never runs

Answer

The effect runs after every render

Answer

The effect runs before the first render

Question 3

What is wrong with this effect?

Accepted Answer

roomId is used but not listed as a dependency, so it stays stuck on the first room

Answer

Nothing

Answer

connect must be called inside a handler

Answer

The cleanup runs too often

Question 4

Why does an effect with an object dependency re-run on every render?

Accepted Answer

A new object is created each render, so it never matches the previous one by reference

Answer

Objects cannot be dependencies

Answer

React ignores object dependencies

Answer

The effect body is impure

Question 5

When does an effect's cleanup function run?

Accepted Answer

Before the effect runs again, and when the component unmounts

Answer

Only when the component unmounts

Answer

After every render regardless of dependencies

Answer

Only in production builds

Question 6

Your component fetches twice in development but once in production. What does that tell you?

Accepted Answer

Strict Mode is double-mounting to check that your effect cleans up after itself

Answer

There is a bug in React

Answer

The dependency array is empty

Answer

You must disable Strict Mode

Question 7

Which of these should NOT be done in an effect?

Accepted Answer

Setting state to a value derived from props

Answer

Subscribing to a WebSocket

Answer

Setting up a timer

Answer

Adding a window event listener