React.memo usage - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is React.memo used for in React?

React.memo is a function that helps to optimize functional components by preventing unnecessary re-renders when the component's props have not changed.

beginner

How do you wrap a functional component with React.memo?

<p>You wrap the component by exporting it as <code>export default React.memo(ComponentName);</code> or by assigning <code>const MemoComponent = React.memo(ComponentName);</code></p>

intermediate

What kind of comparison does React.memo use by default to check if props changed?

It uses a shallow comparison of the props, meaning it checks if the prop values are the same by reference or primitive equality.

intermediate

How can you customize the prop comparison in React.memo?

You can pass a second argument to React.memo, a function that receives the old and new props and returns true if they are equal (skip re-render) or false if not.

intermediate

When should you avoid using React.memo?

Avoid using React.memo if the component is very simple or if props change frequently, because the cost of comparing props might outweigh the benefits.

What does React.memo do?

A Automatically updates state

B Prevents re-render if props are unchanged

C Converts class components to functional

D Handles side effects

React.memo helps skip re-rendering a component if its props have not changed.

How does React.memo compare props by default?

A Deep comparison of all nested objects

B Compares only the first prop

C Ignores props and always re-renders

D Shallow comparison of props

It uses shallow comparison, checking if prop values are the same by reference or primitive equality.

Which is a correct way to use React.memo?

A MyComponent.memo()

B React.memo(MyComponent)();

C const MemoComp = React.memo(MyComponent);

D React.memo = MyComponent;

You wrap the component by assigning React.memo(MyComponent) to a variable or export it directly.

What can you pass as a second argument to React.memo?

A A function to customize prop comparison

B A boolean to force re-render

C An object with styles

D A string with component name

The second argument is a function that compares old and new props to decide if re-render is needed.

When might React.memo not improve performance?

A When props change frequently

B When component is complex

C When props rarely change

D When using class components

If props change often, the cost of comparing props can outweigh the benefits of memoization.

Explain how React.memo helps improve React app performance and when to use it.

Think about how React decides to update components.

Describe how to customize prop comparison in React.memo and why you might want to do that.

Consider cases where shallow comparison is not enough.

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