Effect execution timing in React - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to run the effect only once after the component mounts.

React

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

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

A[null]

B[]

C[true]

D[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-empty dependency 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 makes the effect run only once after the component mounts.

2fill in blank

medium

Complete the code to run the effect whenever the 'count' state changes.

React

useEffect(() => {
  console.log(`Count changed: ${count}`);
}, [1]);

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

A[setCount]

B[]

C[count]

D[count, setCount]

Attempts:

3 left

💡 Hint

Common Mistakes

Using an empty array runs effect only once, not on count changes.

Including setter functions in dependencies is usually unnecessary.

Including count in the dependency array makes the effect run whenever count changes.

3fill in blank

hard

Fix the error in the effect to avoid running infinitely when 'value' changes.

React

useEffect(() => {
  setValue(value + 1);
}, [1]);

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

A[value, setValue]

B[value]

C[setValue]

D[]

Attempts:

3 left

💡 Hint

Common Mistakes

Including 'value' in dependencies causes infinite re-renders.

Not using a dependency array runs effect every render.

Using an empty dependency array prevents the effect from running infinitely by not reacting to value changes.

4fill in blank

hard

Fill both blanks to run the effect when 'userId' changes and clean up before next run.

React

useEffect(() => {
  const subscription = subscribeToUser([1]);
  return () => [2];
}, [userId]);

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

AuserId

Bsubscription.unsubscribe()

Cunsubscribe()

Dsubscription.close()

Attempts:

3 left

💡 Hint

Common Mistakes

Not cleaning up subscriptions causes memory leaks.

Calling wrong cleanup method causes errors.

The effect subscribes using userId and cleans up by calling subscription.unsubscribe() before the next effect run.

5fill in blank

hard

Fill all three blanks to update document title with count and clean up on unmount.

React

useEffect(() => {
  document.title = `Count: $[1]`;
  return () => {
    document.title = [2];
  };
}, [[3]]);

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

Acount

B'React App'

D[]

Attempts:

3 left

💡 Hint

Common Mistakes

Not including count in dependencies causes stale title.

Not resetting title on cleanup leaves wrong title after unmount.

The effect updates the document title with count. On cleanup, it resets the title to 'React App'. The effect runs whenever count changes.

Practice

1. In React, when does the useEffect hook run its effect function by default?

easy

A. Only when the component unmounts

B. Before the component renders

C. Only once when the component mounts

D. After every render of the component

Solution

1. Step 1: Understand default useEffect behavior

   By default, useEffect runs after every render, including the first render and all updates.

2. Step 2: Compare with other lifecycle timings

   It does not run before render or only once unless dependencies are specified.

3. Final Answer:

   After every render of the component -> Option D

4. Quick Check:

   Default effect runs after render [OK]

Hint: Default useEffect runs after every render unless dependencies given [OK]

Common Mistakes:

• Thinking useEffect runs before render
• Assuming it runs only once without dependencies
• Confusing cleanup timing with effect timing

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

easy

A. useEffect(() => { /* effect */ }, [true])

B. useEffect(() => { /* effect */ })

C. useEffect(() => { /* effect */ }, [])

D. useEffect(() => { /* effect */ }, [null])

Solution

1. Step 1: Identify dependency array usage

   An empty array [] means the effect runs only once after the first render (mount).

2. Step 2: Check other options

   Omitting dependencies runs every render; non-empty arrays cause effect to run on changes.

3. Final Answer:

   useEffect(() => { /* effect */ }, []) -> Option C

4. Quick Check:

   Empty dependency array = run once on mount [OK]

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

Common Mistakes:

• Omitting dependency array runs effect every render
• Using non-empty arrays thinking it runs once
• Confusing null or true as empty array

3. Consider this React component:

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

What will be logged to the console after clicking the button twice?

medium

A. Effect: 0, Cleanup: 0, Effect: 1, Cleanup: 1, Effect: 2

B. Effect: 0, Effect: 1, Effect: 2

C. Effect: 0, Cleanup: 0, Cleanup: 1, Effect: 2

D. Cleanup: 0, Cleanup: 1, Effect: 2

Solution

1. Step 1: Understand effect and cleanup with dependencies

   Effect runs after mount and after each count change. Cleanup runs before effect re-runs or unmount.

2. Step 2: Trace logs on two clicks

   Initial mount: Effect: 0
   First click: Cleanup: 0, Effect: 1
   Second click: Cleanup: 1, Effect: 2

3. Final Answer:

   Effect: 0, Cleanup: 0, Effect: 1, Cleanup: 1, Effect: 2 -> Option A

4. Quick Check:

   Effect runs after render; cleanup before next effect [OK]

Hint: Cleanup runs before next effect when dependencies change [OK]

Common Mistakes:

• Ignoring cleanup logs
• Assuming cleanup runs after effect
• Missing initial effect run on mount

4. What is wrong with this React effect code?

React.useEffect(() => {
  const id = setInterval(() => console.log('Tick'), 1000);
  return clearInterval(id);
}, []);

medium

A. The effect will run multiple times because dependencies are missing

B. The cleanup function should be a function, not the result of clearInterval

C. setInterval should be replaced with setTimeout

D. The effect will never run because of missing dependencies

Solution

1. Step 1: Check cleanup function syntax

   The cleanup must be a function returning a function, but here it calls clearInterval immediately.

2. Step 2: Correct cleanup usage

   It should return a function: return () => clearInterval(id); to run cleanup on unmount.

3. Final Answer:

   The cleanup function should be a function, not the result of clearInterval -> Option B

4. Quick Check:

   Cleanup must be a function returning a function [OK]

Hint: Cleanup must be a function, not a direct call [OK]

Common Mistakes:

• Returning cleanup call instead of function
• Confusing dependencies with cleanup
• Thinking setInterval needs setTimeout

5. You want a React effect to run only when either propA or stateB changes, but not on every render. Which dependency array is correct?

React.useEffect(() => {
  // effect code
}, ???);

hard

A. [propA, stateB]

B. []

C. [propA || stateB]

D. [propA && stateB]

Solution

1. Step 1: Understand dependency array purpose

   The effect runs when any value in the array changes compared to last render.

2. Step 2: Analyze options

   [propA, stateB] lists both dependencies separately, so effect runs if either changes.
   Options C and D use logical operators, which evaluate to single values and won't track changes properly.
   [] runs effect only once on mount.

3. Final Answer:

   [propA, stateB] -> Option A

4. Quick Check:

   List all dependencies separately in array [OK]

Hint: List each dependency separately in array to track changes [OK]

Common Mistakes:

• Using logical operators inside dependency array
• Leaving dependency array empty when dependencies exist
• Assuming single combined value tracks all changes