React.memo usage - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output behavior of this React.memo wrapped component?

Consider the following React component wrapped with React.memo. What will happen when the parent component re-renders but the props remain the same?

React

import React, { useState } from 'react';

const Child = React.memo(({ count }) => {
  console.log('Child rendered');
  return <div>Count: {count}</div>;
});

export default function Parent() {
  const [count, setCount] = useState(0);
  const [other, setOther] = useState(0);

  return (
    <>
      <Child count={count} />
      <button onClick={() => setOther(other + 1)}>Change Other</button>
    </>
  );
}

A The Child component only renders once initially and does not re-render when the Parent's 'other' state changes.

B The Child component never renders because React.memo prevents initial rendering.

C The Child component re-renders every time the Parent re-renders, logging 'Child rendered' each time.

D The Child component re-renders only when the Parent's 'other' state changes.

💡 Hint

React.memo prevents re-rendering if props do not change.

React.memo wraps the Child component to memoize it. It only re-renders if its props change. Since 'count' stays the same when 'other' changes, Child does not re-render, so 'Child rendered' logs only once.

📝 Syntax

intermediate

Which option correctly uses React.memo with a custom comparison function?

You want to prevent re-rendering of a component unless the 'user.id' prop changes. Which code snippet correctly uses React.memo with a custom comparison function?

A const MemoComp = React.memo(Component, (prev, next) => prev.user.id > next.user.id);

B const MemoComp = React.memo(Component, (prev, next) => prev.user.id !== next.user.id);

C const MemoComp = React.memo(Component, (prev, next) => prev.user === next.user);

D const MemoComp = React.memo(Component, (prev, next) => prev.user.id === next.user.id);

💡 Hint

The comparison function should return true if props are equal to skip re-render.

The second argument to React.memo is a function that returns true if props are equal (to skip re-render) and false if props changed (to re-render). Option D returns true when user.id is the same, so it is correct.

❓ state_output

advanced

What will be logged when clicking the button in this React.memo example?

Analyze the code below. What will be logged to the console when the button is clicked twice?

React

import React, { useState } from 'react';

const Display = React.memo(({ value }) => {
  console.log('Display rendered with value:', value);
  return <div>{value}</div>;
});

export default function App() {
  const [count, setCount] = useState(0);

  return (
    <>
      <Display value={count} />
      <button onClick={() => setCount(count)}>Click me</button>
    </>
  );
}

A 'Display rendered with value: 0' logs twice, once on initial render and once after each click.

B 'Display rendered with value: 0' logs once initially and once after the first click only.

C 'Display rendered with value: 0' logs only once on initial render.

D No logs appear because the component never renders.

💡 Hint

React.memo skips re-render if props are shallowly equal.

Clicking the button calls setCount with the same value (count). React.memo sees the same prop value and skips re-rendering Display, so the log appears only once on initial render.

🔧 Debug

advanced

Why does this React.memo component still re-render on every parent update?

Look at the code below. Despite using React.memo, the Child component re-renders every time the Parent updates. Why?

React

import React, { useState } from 'react';

const Child = React.memo(({ data }) => {
  console.log('Child rendered');
  return <div>{data.text}</div>;
});

export default function Parent() {
  const [count, setCount] = useState(0);
  const data = { text: 'Hello' };

  return (
    <>
      <Child data={data} />
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </>
  );
}

A Because React.memo does not work with objects as props.

B Because the 'data' object is recreated on every render, causing props to be different.

C Because the Child component is missing a key prop.

D Because the Parent component does not use useCallback for the button handler.

💡 Hint

Objects are compared by reference, not by content.

Each render of Parent creates a new 'data' object. React.memo shallowly compares props by reference, so it sees a new object each time and re-renders Child.

🧠 Conceptual

expert

Which statement best describes React.memo's behavior?

Select the most accurate description of how React.memo works in React functional components.

A React.memo shallowly compares props and skips re-render if props are equal by reference or primitive value.

B React.memo prevents all re-renders of a component regardless of prop changes.

C React.memo deeply compares all nested objects in props to decide on re-rendering.

D React.memo only works with class components to optimize rendering.

💡 Hint

Think about how JavaScript compares objects and primitives.

React.memo performs a shallow comparison of props. It compares primitive values by value and objects by reference. If props are equal, it skips re-rendering the component.

Practice

1.

What is the main purpose of React.memo in React?

easy

A. To manage state inside a functional component

B. To convert a functional component into a class component

C. To prevent unnecessary re-rendering of a functional component when its props have not changed

D. To fetch data from an API automatically

Solution

1. Step 1: Understand React.memo's role

   React.memo is a higher-order component that wraps a functional component to optimize rendering.

2. Step 2: Identify its effect on rendering

   It skips re-rendering the component if the props are the same as the previous render.

3. Final Answer:

   To prevent unnecessary re-rendering of a functional component when its props have not changed -> Option C

4. Quick Check:

   React.memo optimizes rendering = A [OK]

Hint: React.memo skips re-render if props unchanged [OK]

Common Mistakes:

• Thinking React.memo manages state
• Confusing React.memo with class components
• Assuming React.memo fetches data

2.

Which of the following is the correct syntax to wrap a functional component MyComponent with React.memo?

easy

A. const MemoizedComponent = React.memo(MyComponent);

B. const MemoizedComponent = React.memo => MyComponent();

C. const MemoizedComponent = React.memo(MyComponent());

D. const MemoizedComponent = React.memo{MyComponent};

Solution

1. Step 1: Recall React.memo usage syntax

   React.memo is a function that takes a component and returns a memoized version.

2. Step 2: Check each option's syntax

   const MemoizedComponent = React.memo(MyComponent); correctly calls React.memo with the component reference without invoking it.

3. Final Answer:

   const MemoizedComponent = React.memo(MyComponent); -> Option A

4. Quick Check:

   React.memo(Component) syntax = D [OK]

Hint: React.memo takes component, no parentheses after component name [OK]

Common Mistakes:

• Calling component inside React.memo (using parentheses)
• Using arrow function syntax incorrectly
• Using braces instead of parentheses

3.

Consider this code snippet:

const MyComponent = React.memo(({ count }) => {
  console.log('Rendered');
  return <div>Count: {count}</div>;
});

function App() {
  const [count, setCount] = React.useState(0);
  const [text, setText] = React.useState('');

  return (
    <>
      <MyComponent count={count} />
      <input value={text} onChange={e => setText(e.target.value)} />
    </>
  );
}

What will be logged to the console when the user types in the input box?

medium

A. 'Rendered' logs only once when the component mounts

B. 'Rendered' logs every time the input changes

C. 'Rendered' never logs

D. 'Rendered' logs only when count changes

Solution

1. Step 1: Understand React.memo behavior with props

   MyComponent is memoized and only re-renders if its props change. It receives count as prop.

2. Step 2: Analyze prop changes on input update

   Typing changes text state, causing App to re-render. However, the count prop remains unchanged (same primitive value), so React.memo skips re-rendering MyComponent.

3. Step 3: Check if console logs on input change

   Since count does not change, React.memo prevents re-render, so 'Rendered' logs only once on mount.

4. Final Answer:

   'Rendered' logs only once when the component mounts -> Option A

5. Quick Check:

   React.memo skips re-render if props unchanged = B [OK]

Hint: React.memo skips re-render if props unchanged, so no log on input [OK]

Common Mistakes:

• Assuming any parent state change triggers child re-render
• Ignoring React.memo's prop comparison
• Thinking console logs every input change

4.

Consider this usage of React.memo:

const MyComponent = React.memo(({ value }) => {
  return <div>Value: {value}</div>;
}, (prevProps, nextProps) => {
  return prevProps.value === nextProps.value;
});

Which of the following correctly describes this code?

medium

A. The component function should not be an arrow function

B. The custom comparison function should return false when props are equal

C. React.memo cannot accept a second argument

D. The custom comparison function returns true when props are equal, which prevents re-rendering correctly

Solution

1. Step 1: Understand React.memo's custom comparison function

   The second argument is a function that returns true if props are equal, meaning skip re-render.

2. Step 2: Analyze the provided function

   The function returns true when prevProps.value === nextProps.value, which is correct to skip re-render when props are equal.

3. Step 3: Confirm the correct description

   The custom comparison function returns true when props are equal, which prevents re-rendering correctly accurately describes that it returns true when props are equal, preventing re-rendering correctly.

4. Final Answer:

   The custom comparison function returns true when props are equal, which prevents re-rendering correctly -> Option D

5. Quick Check:

   Custom compare returns true to skip re-render = C [OK]

Hint: Custom compare returns true to skip re-render [OK]

Common Mistakes:

• Thinking custom compare should return false when props equal
• Believing React.memo can't take a second argument
• Confusing arrow function usage

5.

You have a component wrapped with React.memo that receives a prop data which is an object. You notice it re-renders every time the parent renders, even if data has the same content. How can you fix this?

hard

A. Convert data to a string before passing it as prop

B. Use a custom comparison function in React.memo to do a deep comparison of data

C. Wrap the parent component with React.memo instead

D. Use useState inside the child component to store data

Solution

1. Step 1: Understand why re-renders happen with object props

   Objects are compared by reference, so even if content is same, new object causes re-render.

2. Step 2: Use custom comparison function

   React.memo accepts a second argument to compare props deeply and prevent re-render if content is equal.

3. Step 3: Evaluate other options

   Converting to string is inefficient and error-prone; wrapping parent doesn't solve child re-render; useState inside child doesn't prevent re-render.

4. Final Answer:

   Use a custom comparison function in React.memo to do a deep comparison of data -> Option B

5. Quick Check:

   Deep compare object props with custom function = A [OK]

Hint: Use custom compare to deep check object props [OK]

Common Mistakes:

• Passing new object each render without memoization
• Ignoring object reference equality
• Trying to fix with useState inside child