Common lifecycle use cases in React - Cheat Sheet & Quick Revision

Recall & Review

beginner

What React hook is used to run code when a component first appears on the screen?

The useEffect hook with an empty dependency array [] runs code once when the component mounts.

intermediate

How do you clean up resources like timers or subscriptions in React functional components?

Return a cleanup function inside useEffect. This function runs when the component unmounts or before the effect runs again.

intermediate

What is a common use case for running code when a component updates?

Use useEffect with specific dependencies to run code only when those values change, like fetching new data when a prop changes.

advanced

Why should you avoid running expensive code on every render in React?

Because it slows down the app. Use useMemo or useCallback to remember results and avoid unnecessary work.

advanced

How can you simulate componentDidMount, componentDidUpdate, and componentWillUnmount in React functional components?

Use useEffect: with empty dependencies for mount, with dependencies for update, and return a cleanup function for unmount.

Which React hook runs after every render by default?

A useMemo

B useState

C useEffect without dependencies

D useRef

useEffect without a dependency array runs after every render.

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

A useEffect with empty dependency array []

B useEffect with dependencies

C useEffect with no arguments

D useState

An empty dependency array [] tells useEffect to run only once on mount.

What does the cleanup function inside useEffect do?

A Runs after every render

B Runs before the effect runs again or when component unmounts

C Runs only on mount

D Runs only on update

The cleanup function runs before the next effect or when the component unmounts.

Which hook helps avoid running expensive calculations on every render?

A useRef

B useState

C useEffect

D useMemo

useMemo memoizes values to avoid recalculating on every render.

To run code when a specific prop changes, you should:

A Use useEffect with that prop in the dependency array

B Use useState

C Use useRef

D Use useEffect with empty dependencies

Including the prop in useEffect's dependency array runs the effect when it changes.

Explain how to use the useEffect hook to handle component mount, update, and unmount in React functional components.

Think about when useEffect runs and how dependencies control it.

Describe why and how you would clean up resources like timers or subscriptions in React components.

Consider what happens if you don't clean up after effects.

Practice

1. What is the primary purpose of the useEffect hook in React functional components?

easy

A. To define event handlers

B. To declare state variables

C. To run code after the component renders or updates

D. To style components dynamically

Solution

1. Step 1: Understand the role of useEffect

   The useEffect hook runs side effects like data fetching or timers after rendering.

2. Step 2: Compare with other options

   Declaring state is done with useState, event handlers are functions, and styling is separate.

3. Final Answer:

   To run code after the component renders or updates -> Option C

4. Quick Check:

   useEffect runs side effects after render [OK]

Hint: useEffect runs code after render or update [OK]

Common Mistakes:

• Confusing useEffect with useState
• Thinking useEffect is for styling
• Believing useEffect defines event handlers

2. Which of the following is the correct syntax to run an effect only once when a React component mounts?

easy

A. useEffect(() => { console.log('Mounted'); });

B. useEffect(() => { console.log('Mounted'); }, []);

C. useEffect(() => { console.log('Mounted'); }, [() => {}]);

D. useEffect(() => { console.log('Mounted'); }, [{}]);

Solution

1. Step 1: Understand dependency array in useEffect

   An empty array [] means the effect runs once on mount only.

2. Step 2: Analyze other options

   Omitting dependencies runs effect after every render; arrays with unstable values like functions or objects run on every render since they are recreated.

3. Final Answer:

   useEffect(() => { console.log('Mounted'); }, []); -> Option B

4. Quick Check:

   Empty array runs effect once on mount [OK]

Hint: Empty dependency array means run once on mount [OK]

Common Mistakes:

• Omitting dependency array causes repeated runs
• Using non-empty arrays runs effect multiple times
• Putting recreated functions or objects in dependency array causes re-runs

3. Consider this React component:

function Timer() {
  const [count, setCount] = React.useState(0);
  React.useEffect(() => {
    const id = setInterval(() => {
      setCount(c => c + 1);
    }, 1000);
    return () => clearInterval(id);
  }, []);
  return <div>Count: {count}</div>;
}

What will be displayed after 3 seconds?

medium

A. Count: 0

B. Count: 1

C. Count: 1000

D. Count: 3

Solution

1. Step 1: Understand the interval effect

   The effect sets an interval that increments count every 1000ms (1 second).

2. Step 2: Calculate count after 3 seconds

   After 3 intervals, count increments from 0 to 3.

3. Final Answer:

   Count: 3 -> Option D

4. Quick Check:

   Interval increments count every second [OK]

Hint: Interval increments count every second [OK]

Common Mistakes:

• Ignoring cleanup function
• Thinking count stays 0
• Confusing milliseconds with count value

4. What is wrong with this React useEffect usage?

useEffect(() => {
  const timer = setTimeout(() => {
    console.log('Hello');
  }, 1000);
});

medium

A. Missing dependency array causes multiple timeouts to be set on every render

B. setTimeout is not allowed inside useEffect

C. console.log cannot be used inside useEffect

D. The timeout duration must be less than 500ms

Solution

1. Step 1: Check dependency array presence

   Without a dependency array, the effect runs after every render, creating a new timeout each time.

2. Step 2: Understand consequences

   This causes multiple timers to accumulate (no cleanup returned), leading to multiple console.logs firing.

3. Final Answer:

   Missing dependency array causes multiple timeouts to be set on every render -> Option A

4. Quick Check:

   Missing dependencies cause repeated effect runs [OK]

Hint: Always add dependency array to avoid repeated effects [OK]

Common Mistakes:

• Thinking setTimeout is disallowed in useEffect
• Believing console.log can't be used
• Assuming timeout duration limits

5. You want to fetch user data only when the userId prop changes in a React component. Which useEffect hook is correct?

function UserProfile({ userId }) {
  const [user, setUser] = React.useState(null);

  useEffect(() => {
    fetch(`https://api.example.com/users/${userId}`)
      .then(res => res.json())
      .then(data => setUser(data));
  }, ???);

  return <div>{user ? user.name : 'Loading...'}</div>;
}

hard

A. [userId]

B. [user]

C. []

D. [fetch]

Solution

1. Step 1: Identify when to run the effect

   The effect should run only when userId changes to fetch new data.

2. Step 2: Choose correct dependency array

   Including userId in the dependency array triggers effect on its change.

3. Final Answer:

   [userId] -> Option A

4. Quick Check:

   Dependency array controls when effect runs [OK]

Hint: Put changing props in dependency array [OK]

Common Mistakes:

• Using empty array runs effect only once
• Using state variable instead of prop
• Including functions like fetch in dependencies