React.memo usage - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to wrap the component with React.memo.

React

const MemoizedComponent = [1](MyComponent);

Drag options to blanks, or click blank then click option'

AReact.useEffect

BReact.memo

CReact.useState

DReact.createContext

Attempts:

3 left

💡 Hint

Common Mistakes

Using React.useState or React.useEffect instead of React.memo

Trying to call React.memo without parentheses

React.memo is used to wrap a component to memoize it and avoid unnecessary re-renders.

2fill in blank

medium

Complete the code to export the memoized component as default.

React

export default [1](MyComponent);

Drag options to blanks, or click blank then click option'

AReact.memo

BReact.useCallback

CReact.forwardRef

DReact.lazy

Attempts:

3 left

💡 Hint

Common Mistakes

Using React.useCallback which memoizes functions, not components

Using React.forwardRef which is for refs, not memoization

Exporting the component wrapped in React.memo ensures it is memoized when imported elsewhere.

3fill in blank

hard

Fix the error in the code to correctly memoize the component.

React

const MyComponent = (props) => { return <div>{props.text}</div>; };
const MemoComp = React.[1](MyComponent);

Drag options to blanks, or click blank then click option'

Amemo

BuseMemo

Cmemorize

Dmemoize

Attempts:

3 left

💡 Hint

Common Mistakes

Using misspelled versions like memoize or memorize

Confusing React.memo with React.useMemo

The correct React API is React.memo. Other options are invalid or different hooks.

4fill in blank

hard

Fill both blanks to create a memoized component with a custom comparison function.

React

const MemoComp = React.memo(MyComponent, [1]);

function [2](prevProps, nextProps) {
  return prevProps.value === nextProps.value;
}

Drag options to blanks, or click blank then click option'

AareEqual

BcompareProps

CarePropsEqual

DcheckProps

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the function and the argument

Passing a non-function value as the second argument

The custom comparison function is passed as the second argument to React.memo. Naming it areEqual is common and matches usage.

5fill in blank

hard

Fill all three blanks to memoize a component and export it with a custom comparison function.

React

function [1](prev, next) {
  return prev.count === next.count;
}

const MemoComp = React.[2](Counter, [3]);

export default MemoComp;

Drag options to blanks, or click blank then click option'

AareEqual

Bmemo

DuseMemo

Attempts:

3 left

💡 Hint

Common Mistakes

Using useMemo instead of memo

Mismatching function names between declaration and argument

The function areEqual is the custom comparison passed to React.memo to memoize the component with custom logic.

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