Lifecycle mapping with hooks in React - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to run a function after the component mounts.

React

useEffect(() => {
  console.log('Component mounted');
}, [1]);

Drag options to blanks, or click blank then click option'

A[]

B[true]

C[null]

D[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-empty array causes the effect to run multiple times.

Leaving out the dependency array runs the effect after every render.

Using an empty array [] as the dependency list runs the effect only once after the component mounts.

2fill in blank

medium

Complete the code to clean up a subscription when the component unmounts.

React

useEffect(() => {
  const id = subscribe();
  return () => {
    [1](id);
  };
}, []);

Drag options to blanks, or click blank then click option'

Asubscribe

Bunsubscribe

Cclear

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Calling subscribe again in cleanup causes duplicate subscriptions.

Not returning a cleanup function causes resource leaks.

The cleanup function should call unsubscribe to stop the subscription when the component unmounts.

3fill in blank

hard

Fix the error in the effect to update the document title when count changes.

React

useEffect(() => {
  document.title = `Count: ${count}`;
}, [1]);

Drag options to blanks, or click blank then click option'

A[count]

B[title]

C[setCount]

D[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using an empty array means the title never updates after first render.

Using unrelated variables in dependencies causes unnecessary runs.

The effect should depend on count so it runs whenever count changes.

4fill in blank

hard

Fill both blanks to create a state and update it on button click.

React

const [[1], [2]] = useState(0);

<button onClick={() => [2]([1] + 1)}>Increment</button>

Drag options to blanks, or click blank then click option'

Acount

BsetCount

Cvalue

DsetValue

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the order of state and setter.

Using unrelated names that don't match in update function.

The first blank is the state variable name, and the second is the function to update it.

5fill in blank

hard

Fill all three blanks to fetch data on mount and store it in state.

React

const [[1], [2]] = useState(null);

useEffect(() => {
  fetch('/api/data')
    .then(res => res.json())
    .then(data => [2](data));
}, [3]);

Drag options to blanks, or click blank then click option'

Adata

BsetData

C[]

D[data]

Attempts:

3 left

💡 Hint

Common Mistakes

Not using the setter to update state.

Missing or wrong dependency array causing multiple fetches.

We create a state variable data and its setter setData. The effect runs once on mount with an empty dependency array [].

Practice

1. Which React hook is used to run code after a component renders, replacing lifecycle methods like componentDidMount and componentDidUpdate?

easy

A. useEffect

B. useState

C. useContext

D. useRef

Solution

1. Step 1: Understand lifecycle replacement

   The useEffect hook runs code after rendering, replacing lifecycle methods like componentDidMount and componentDidUpdate.

2. Step 2: Identify the correct hook

   useState manages state, useContext accesses context, and useRef stores mutable values. Only useEffect runs side effects after render.

3. Final Answer:

   useEffect -> Option A

4. Quick Check:

   Effect hook = useEffect [OK]

Hint: useEffect runs code after render like lifecycle methods [OK]

Common Mistakes:

• Confusing useState with useEffect
• Thinking useRef runs effects
• Believing useContext triggers after render

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

easy

A. useEffect(() => { /* code */ }, null);

B. useEffect(() => { /* code */ });

C. useEffect(() => { /* code */ }, true);

D. useEffect(() => { /* code */ }, []);

Solution

1. Step 1: Understand dependency array usage

   Passing an empty array [] as the second argument to useEffect makes it run only once after the first render (component mount).

2. Step 2: Analyze each option

   useEffect(() => { /* code */ }); runs after every render. useEffect(() => { /* code */ }, true); runs after every render since non-array dependencies are ignored. useEffect(() => { /* code */ }, null); with null also runs after every render as it's not an array.

3. Final Answer:

   useEffect(() => { /* code */ }, []); -> Option D

4. Quick Check:

   Empty array = run once [OK]

Hint: Empty dependency array runs effect once on mount [OK]

Common Mistakes:

• Omitting dependency array causes effect every render
• Using null instead of empty array
• Passing non-array values like true or null as dependencies

3. What will be logged to the console when the following React component mounts and then the count state changes?

function Counter() {
  const [count, setCount] = React.useState(0);
  React.useEffect(() => {
    console.log('Count is', count);
  }, [count]);
  return  setCount(count + 1)}>Increment;
}

medium

A. Logs 'Count is 0' once on mount only

B. Logs 'Count is 0' repeatedly every second

C. Logs 'Count is 0' on mount and 'Count is 1' after first click

D. Logs nothing because useEffect has dependencies

Solution

1. Step 1: Understand useEffect with dependency

   The effect runs after the component mounts and whenever count changes because count is in the dependency array.

2. Step 2: Trace the console logs

   On mount, count is 0, so it logs 'Count is 0'. After clicking the button, count becomes 1, triggering the effect again and logging 'Count is 1'.

3. Final Answer:

   Logs 'Count is 0' on mount and 'Count is 1' after first click -> Option C

4. Quick Check:

   Effect runs on mount and dependency change [OK]

Hint: Effect runs on mount and when dependencies change [OK]

Common Mistakes:

• Thinking effect runs only once without dependencies
• Assuming no logs because dependencies exist
• Confusing useEffect with setInterval

4. Identify the problem in this React component that uses useEffect and suggest the fix:

function Timer() {
  React.useEffect(() => {
    const id = setInterval(() => console.log('Tick'), 1000);
  }, []);
  return <div>Timer</div>;
}

medium

A. No problem; code works fine as is

B. Missing cleanup function to clear the interval on unmount

C. Dependency array should include 'id' to update interval

D. setInterval should be replaced with setTimeout

Solution

1. Step 1: Analyze effect side effects

   The effect sets an interval but does not clear it, causing a memory leak when the component unmounts.

2. Step 2: Suggest cleanup function

   Return a cleanup function from useEffect that calls clearInterval(id) to stop the interval on unmount.

3. Final Answer:

   Missing cleanup function to clear the interval on unmount -> Option B

4. Quick Check:

   Cleanup prevents memory leaks [OK]

Hint: Always clean intervals/timeouts in useEffect cleanup [OK]

Common Mistakes:

• Forgetting to clear intervals causing leaks
• Adding dependencies unnecessarily
• Replacing setInterval with setTimeout incorrectly

5. You want to fetch user data only once when a component mounts and cancel the fetch if the component unmounts before it finishes. Which is the best way to implement this using useEffect?

hard

A. useEffect(() => {
const controller = new AbortController();
fetch('/user', { signal: controller.signal })
.then(res => res.json())
.then(data => console.log(data));
return () => controller.abort();
}, []);

B. useEffect(() => {
fetch('/user').then(res => res.json()).then(data => console.log(data));
}, []);

C. useEffect(() => {
fetch('/user').then(res => res.json()).then(data => console.log(data));
return () => console.log('Cleanup');
}, []);

D. useEffect(() => {
fetch('/user').then(res => res.json()).then(data => console.log(data));
}, [fetch]);

Solution

1. Step 1: Understand fetch cancellation

   To cancel a fetch request on unmount, use an AbortController and pass its signal to fetch.

2. Step 2: Implement cleanup to abort fetch

   Return a cleanup function from useEffect that calls controller.abort() to cancel the fetch if unmount happens early.

3. Final Answer:

   Use AbortController with cleanup to abort fetch -> Option A

4. Quick Check:

   AbortController + cleanup = safe fetch [OK]

Hint: Use AbortController and cleanup to cancel fetch on unmount [OK]

Common Mistakes:

• Not cancelling fetch causing memory leaks
• Logging cleanup without aborting fetch
• Adding fetch to dependencies unnecessarily