Lifecycle mapping with hooks in React - Cheat Sheet & Quick Revision

Recall & Review

beginner

What React hook is used to run code after the component first renders?

The <code>useEffect</code> hook with an empty dependency array (<code>[]</code>) runs code after the first render, similar to <code>componentDidMount</code> in class components.

beginner

How do you run cleanup code when a React component unmounts using hooks?

Inside useEffect, return a function. This returned function runs when the component unmounts, similar to componentWillUnmount.

intermediate

Which hook replaces componentDidUpdate behavior?

The useEffect hook runs after every render by default. To mimic componentDidUpdate, use useEffect with specific dependencies to run code when those values change.

beginner

What does the dependency array in useEffect control?

The dependency array tells React when to re-run the effect. If empty ([]), it runs once after mount. If it has variables, it runs when those variables change.

intermediate

How can you run code only when a specific prop or state changes in a functional component?

Use useEffect with that prop or state in the dependency array. React runs the effect only when those values change.

Which hook is primarily used to handle lifecycle events in React functional components?

A useState

B useRef

C useContext

D useEffect

useEffect is used to run code on mount, update, and unmount, replacing lifecycle methods.

What happens if you omit the dependency array in useEffect?

A Effect runs only once after mount

B Effect runs after every render

C Effect never runs

D Effect runs only on unmount

Without dependencies, useEffect runs after every render.

How do you run cleanup code when a component unmounts?

A Return a function inside <code>useEffect</code>

B Use <code>useState</code> to reset values

C Call cleanup in the component body

D Use <code>useRef</code> to track unmount

Returning a function inside useEffect runs that function on unmount.

To run an effect only when a specific state changes, what should you do?

A Do not use <code>useEffect</code>

B Leave the dependency array empty

C Add that state to the dependency array

D Use <code>useState</code> instead

Adding the state to the dependency array triggers the effect only when it changes.

Which lifecycle method does useEffect(() => {}, []) mimic?

A componentDidMount

B componentWillUnmount

C componentDidUpdate

D render

An empty dependency array runs the effect once after the component mounts.

Explain how you can use React hooks to mimic the lifecycle methods: componentDidMount, componentDidUpdate, and componentWillUnmount.

Think about how useEffect runs after render and how dependencies control when it runs.

Describe the role of the dependency array in the useEffect hook and how it affects when the effect runs.

Consider how React decides when to re-run the effect.

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