Mounting phase in React - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the React hook used to run code during the mounting phase.

React

import React, { [1] } from 'react';

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

AuseRef

BuseState

CuseContext

DuseEffect

Attempts:

3 left

💡 Hint

Common Mistakes

Using useState instead of useEffect for running code on mount.

Confusing useRef with useEffect.

The useEffect hook runs code during the mounting phase in React functional components.

2fill in blank

medium

Complete the code to run a console log only once when the component mounts.

React

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

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

A[]

B[0]

C[null]

D[true]

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving the dependency array empty (undefined) causes the effect to run on every render.

Using non-empty arrays causes multiple runs.

An empty dependency array [] makes useEffect run only once when the component mounts.

3fill in blank

hard

Fix the error in the useEffect hook to avoid running the effect multiple times.

React

useEffect(() => {
  fetchData();
}, [1]);

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

A[true]

B[]

C[fetchData]

D[data]

Attempts:

3 left

💡 Hint

Common Mistakes

Adding variables that change often to the dependency array causing repeated runs.

Omitting the dependency array causing infinite loops.

Using an empty array [] ensures fetchData runs only once on mount, preventing repeated calls.

4fill in blank

hard

Fill both blanks to correctly set up a state and run an effect on mount that updates it.

React

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

useEffect(() => {
  [2](5);
}, []);

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

AsetCount

BsetValue

Ccount

DsetNumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using the state variable instead of the setter function to update state.

Using a setter function name that was not declared.

The setter function from useState is setCount. We use it inside useEffect to update the state on mount.

5fill in blank

hard

Fill all three blanks to create a component that logs on mount and cleans up on unmount.

React

import React, { [1] } from 'react';

function Logger() {
  [2](() => {
    console.log('Mounted');
    return () => {
      console.log([3]);
    };
  }, []);

  return <div>Logger Component</div>;
}

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

AuseEffect

BuseState

C'Unmounted'

D'Component unmounted'

Attempts:

3 left

💡 Hint

Common Mistakes

Using useState instead of useEffect for side effects.

Logging the wrong message in the cleanup function.

useEffect runs code on mount and can return a cleanup function that runs on unmount. The cleanup logs 'Component unmounted'.

Practice

1. What does the mounting phase in React mean?

easy

A. When a component updates due to state change

B. When a component first appears on the screen

C. When a component is removed from the screen

D. When a component re-renders after props change

Solution

1. Step 1: Understand React component lifecycle phases

   The mounting phase is the first phase when a component is created and inserted into the DOM.

2. Step 2: Match the phase to the description

   Mounting means the component appears on screen for the first time, not updates or removal.

3. Final Answer:

   When a component first appears on the screen -> Option B

4. Quick Check:

   Mounting phase = first appearance [OK]

Hint: Mounting means first time component shows [OK]

Common Mistakes:

• Confusing mounting with updating
• Thinking mounting means removal
• Mixing mounting with re-rendering

2. Which of the following is the correct way to run code only once when a React component mounts using hooks?

easy

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

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

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

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

Solution

1. Step 1: Recall useEffect dependency array usage

   Passing an empty array [] runs the effect only once on mount.

2. Step 2: Compare options

   useEffect(() => { /* code */ }, []) uses [] so runs once; others run on every render or when dependencies change.

3. Final Answer:

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

4. Quick Check:

   Empty array means run once on mount [OK]

Hint: Empty array in useEffect runs once on mount [OK]

Common Mistakes:

• Omitting dependency array runs effect every render
• Using state or props array runs effect multiple times
• Confusing useEffect with useState

3. What will be logged to the console when this component mounts?

function Timer() {
  React.useEffect(() => {
    console.log('Timer started');
  }, []);
  return <div>Timer</div>;
}

medium

A. Timer started every render

B. Nothing is logged

C. Timer started

D. Error: useEffect missing dependencies

Solution

1. Step 1: Analyze useEffect with empty dependency array

   The effect runs once on mount, logging 'Timer started' once.

2. Step 2: Confirm no repeated logs or errors

   No dependencies means no repeated logs; no error occurs.

3. Final Answer:

   Timer started -> Option C

4. Quick Check:

   useEffect with [] logs once on mount [OK]

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

Common Mistakes:

• Thinking it logs every render
• Expecting no logs without return
• Assuming error due to missing dependencies

4. Identify the error in this React component related to the mounting phase:

function DataFetcher() {
  React.useEffect(() => {
    fetch('/api/data')
      .then(response => response.json())
      .then(data => console.log(data));
  });
  return <div>Loading...</div>;
}

medium

A. Missing dependency array causes fetch on every render

B. fetch is called outside useEffect

C. No return statement in component

D. Cannot use fetch inside useEffect

Solution

1. Step 1: Check useEffect dependencies

   Without a dependency array, useEffect runs after every render, causing repeated fetch calls.

2. Step 2: Identify correct fix

   Adding an empty array [] runs fetch only once on mount, preventing repeated calls.

3. Final Answer:

   Missing dependency array causes fetch on every render -> Option A

4. Quick Check:

   No [] means effect runs every render [OK]

Hint: Always add [] to run effect once on mount [OK]

Common Mistakes:

• Forgetting dependency array in useEffect
• Thinking fetch outside useEffect is needed
• Assuming useEffect cannot contain async calls

5. You want to fetch user data only once when a component mounts and store it in state. Which code correctly implements this mounting phase behavior?

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

  // Which useEffect is correct?

  A) React.useEffect(() => {
       fetch('/user')
         .then(res => res.json())
         .then(data => setUser(data));
     }, []);

  B) React.useEffect(() => {
       fetch('/user')
         .then(res => res.json())
         .then(data => setUser(data));
     });

  C) React.useEffect(() => {
       fetch('/user')
         .then(res => res.json())
         .then(data => setUser(data));
     }, [user]);

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

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

hard

A. useEffect with setUser dependency causing repeated fetch

B. useEffect without dependencies fetching every render

C. useEffect with user dependency causing infinite fetch

D. useEffect with empty array to fetch once on mount

Solution

1. Step 1: Understand useEffect dependencies for fetching

   Empty array [] runs effect once on mount; no dependencies cause repeated fetches.

2. Step 2: Analyze each option

   The useEffect with an empty dependency array fetches data only once on mount. Without any dependency array, the effect runs after every render. Depending on user causes an infinite loop since updating user triggers re-renders. Depending on setUser runs only once because setUser is stable, but it is unnecessary and may cause ESLint warnings.

3. Final Answer:

   useEffect with empty array to fetch once on mount -> Option D

4. Quick Check:

   Empty [] means fetch once on mount [OK]

Hint: Empty array in useEffect fetches once on mount [OK]

Common Mistakes:

• Leaving out dependency array causes infinite fetch
• Using state as dependency causing loops
• Misunderstanding stable functions as dependencies