useEffect hook in React Native - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the useEffect hook in React Native?

The useEffect hook lets you run code after the component renders. It helps manage side effects like fetching data, setting up subscriptions.

beginner

How do you run an effect only once when a component mounts using useEffect?

Pass an empty array [] as the second argument to useEffect. This tells React to run the effect only once after the first render.

intermediate

What happens if you omit the dependency array in useEffect?

The effect runs after every render, which can cause performance issues or unwanted repeated actions.

intermediate

How can you clean up side effects in useEffect?

Return a cleanup function inside useEffect. This function runs before the effect runs again or when the component unmounts, useful for removing subscriptions or timers.

intermediate

Explain the role of the dependency array in useEffect.

The dependency array tells React when to re-run the effect. The effect runs only if one of the dependencies changes between renders.

What does passing an empty array [] as the second argument to useEffect do?

A Runs the effect only when the component unmounts

B Runs the effect after every render

C Prevents the effect from running at all

D Runs the effect only once after the first render

An empty dependency array means the effect runs once after the component mounts.

Where should you put cleanup code for subscriptions or timers in useEffect?

A Inside the main body of the effect

B Outside the component

C In a function returned from the effect

D In the dependency array

Returning a function from useEffect lets React clean up before re-running or unmounting.

What happens if you omit the dependency array in useEffect?

A Effect runs after every render

B Effect runs only once

C Effect never runs

D Effect runs only on unmount

Without dependencies, the effect runs after every render.

Which of these is NOT a typical use case for useEffect?

A Defining a component's state

B Setting up a timer

C Setting up event listeners

D Fetching data from an API

State is defined with useState, not useEffect.

If you want an effect to run when a specific variable changes, where do you list that variable?

A Inside the effect function

B In the dependency array

C In the cleanup function

D In the component's props

Variables that trigger the effect to re-run go in the dependency array.

Describe how the useEffect hook works and why it is useful in React Native.

Think about when and why you need to do things like fetch data or set timers.

Explain the importance of the dependency array in useEffect and what happens if it is empty or missing.

Consider how React Native decides when to run your effect.