Common lifecycle use cases in React - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when the component mounts and updates?

Consider this React functional component using hooks:

import React, { useState, useEffect } from 'react';

function Counter() {
  const [count, setCount] = useState(0);

  useEffect(() => {
    console.log('Effect runs');
  });

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

What will be logged to the console when the component first appears and each time the button is clicked?

React

import React, { useState, useEffect } from 'react';

function Counter() {
  const [count, setCount] = useState(0);

  useEffect(() => {
    console.log('Effect runs');
  });

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

A Logs 'Effect runs' once when mounted only.

B Logs 'Effect runs' every time the component renders, including mount and updates.

C Logs nothing because useEffect has no dependencies.

D Logs 'Effect runs' only when the count changes.

💡 Hint

Think about when useEffect runs if no dependency array is provided.

Without a dependency array, useEffect runs after every render, including the first mount and every update.

❓ state_output

intermediate

What is the displayed count after clicking the button once?

Look at this React component:

import React, { useState } from 'react';

function Clicker() {
  const [count, setCount] = useState(0);

  function handleClick() {
    setCount(count + 1);
    setCount(count + 1);
  }

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={handleClick}>Click me twice</button>
    </div>
  );
}

What will the count show after clicking the button once?

React

import React, { useState } from 'react';

function Clicker() {
  const [count, setCount] = useState(0);

  function handleClick() {
    setCount(count + 1);
    setCount(count + 1);
  }

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={handleClick}>Click me twice</button>
    </div>
  );
}

A Count will be 1 after one click.

B Count will be 2 after one click.

C Count will be 0 after one click.

D Count will be 3 after one click.

💡 Hint

Remember that state updates may be batched and use the current state value.

Both setCount calls use the same stale count value (0), so the count increments only once to 1.

🔧 Debug

advanced

Why does this cleanup function not run on unmount?

Consider this React component:

import React, { useEffect } from 'react';

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

  return <div>Timer running</div>;
}

But when the component unmounts, 'tick' keeps logging every second. Why?

React

import React, { useEffect } from 'react';

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

  return <div>Timer running</div>;
}

A The component never unmounts, so cleanup never runs.

B The cleanup function is not returned properly from useEffect.

C The setInterval id is not accessible inside the cleanup function.

D The dependency array is empty, so cleanup never runs.

💡 Hint

Think about when cleanup functions run in React.

Cleanup functions run only when the component unmounts or before re-running the effect. If the component stays mounted, cleanup won't run.

📝 Syntax

advanced

Which option correctly uses useEffect to fetch data only once?

Which code snippet correctly fetches data only once when the component mounts?

A useEffect(fetchData, []);

B useEffect(() => { fetchData(); });

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

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

💡 Hint

Consider how dependency arrays control effect execution.

Option D runs fetchData once on mount because of the empty dependency array. Option D runs on every render. Option D runs when fetchData changes (usually every render if not memoized). Option D passes fetchData directly, which is not a function call and may cause unexpected behavior.

🧠 Conceptual

expert

What happens if you update state inside useEffect without dependencies?

Consider this React component:

import React, { useState, useEffect } from 'react';

function Loop() {
  const [count, setCount] = useState(0);

  useEffect(() => {
    setCount(count + 1);
  });

  return <div>Count: {count}</div>;
}

What will happen when this component renders?

React

import React, { useState, useEffect } from 'react';

function Loop() {
  const [count, setCount] = useState(0);

  useEffect(() => {
    setCount(count + 1);
  });

  return <div>Count: {count}</div>;
}

A The count will stay at 0 because useEffect does not run.

B The count will increment once and stop.

C The component will enter an infinite render loop and crash.

D The component will render twice and then stop.

💡 Hint

Think about what happens when state updates trigger re-renders inside useEffect with no dependencies.

Without a dependency array, useEffect runs after every render. Updating state inside it causes another render, triggering useEffect again, causing an infinite loop.

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