Lifecycle mapping with hooks in React

Introduction

Lifecycle mapping with hooks helps you run code at specific times in a React component's life, like when it starts or updates.

You want to fetch data when a component first appears on the screen.

You need to update the document title when some data changes.

You want to clean up timers or subscriptions when a component disappears.

You want to run code only once when the component loads.

You want to react to changes in specific data or props.

Syntax

React

useEffect(() => {
  // code to run
  return () => {
    // cleanup code
  };
}, [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('Runs once on mount');
}, []);

This runs every time the count value changes.

React

useEffect(() => {
  console.log('Runs when count changes:', count);
}, [count]);

This sets up a timer when the component mounts and cleans it up 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 updates the page title with the count. The timer stops when the component disappears.

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);
  }, []);

  useEffect(() => {
    document.title = `Seconds: ${seconds}`;
  }, [seconds]);

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

export default Timer;

Important Notes

Always clean up side effects like timers or subscriptions to avoid bugs.

Empty dependency array [] means run once on mount and clean on unmount.

Adding variables in dependencies runs effect when those variables change.

Summary

useEffect replaces lifecycle methods by running code after render.

Use dependencies to control when effects run again.

Clean up inside useEffect to avoid memory leaks.

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