Common lifecycle use cases in React

Introduction

Lifecycle methods help you run code at specific times in a component's life. This lets you set up, update, or clean up things smoothly.

Load data from a server when a component appears on the screen.

Set up a timer or interval when a component starts and clear it when it stops.

Listen for window size changes and update the layout accordingly.

Clean up subscriptions or event listeners when a component is removed.

Run code only once when the component first shows.

Syntax

React

useEffect(() => {
  // code to run on mount or update
  return () => {
    // cleanup code runs on unmount or before update
  };
}, [dependencies]);

useEffect runs after the component renders.

The dependencies array controls when the effect runs again.

Examples

This runs only once when the component first appears.

React

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

This runs every time the count value changes.

React

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

This sets up a timer when the component mounts and clears it when it unmounts.

React

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

Sample Program

This component counts seconds after it appears. It starts a timer that updates the count every second. When the component disappears, it stops the timer to avoid problems.

React

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

function Timer() {
  const [seconds, setSeconds] = useState(0);

  useEffect(() => {
    const interval = setInterval(() => {
      setSeconds(s => s + 1);
    }, 1000);

    return () => clearInterval(interval);
  }, []);

  return (
    <div>
      <p>Seconds passed: {seconds}</p>
    </div>
  );
}

export default Timer;

Important Notes

Always clean up timers or listeners in the return function to avoid memory leaks.

Empty dependency array [] means the effect runs only once on mount.

Adding variables to dependencies makes the effect run when those variables change.

Summary

useEffect lets you run code at key moments in a component's life.

Use it to load data, set timers, or clean up resources.

Remember to manage dependencies and cleanup carefully.

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