Avoiding unnecessary renders in React - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a state variable using React hooks.

React

const [count, [1]] = useState(0);

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

AcountSet

BupdateCount

CsetCount

DchangeCount

Attempts:

3 left

💡 Hint

Common Mistakes

Using a name that does not start with 'set' for the update function.

Swapping the order of the state and update function.

The useState hook returns a pair: the current state and a function to update it. The convention is to name the update function as set plus the state variable name.

2fill in blank

medium

Complete the code to memoize a value and avoid recalculating it on every render.

React

const doubled = useMemo(() => count * 2, [[1]]);

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

Acount

Bdoubled

CsetCount

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving the dependency array empty, causing stale values.

Including the memoized variable itself, causing infinite loops.

The dependency array tells React when to recalculate the memoized value. It should include the variables that affect the calculation, here count.

3fill in blank

hard

Fix the error in the code to prevent unnecessary re-renders by wrapping the callback with the correct hook.

React

const increment = [1](() => setCount(count + 1), [count]);

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

AuseCallback

BuseState

CuseMemo

DuseEffect

Attempts:

3 left

💡 Hint

Common Mistakes

Using useEffect instead of useCallback.

Not wrapping the function at all, causing new function instances each render.

useCallback returns a memoized version of the callback that only changes if its dependencies change. This helps avoid unnecessary re-renders when passing callbacks to child components.

4fill in blank

hard

Fill both blanks to create a memoized component that only re-renders when its props change.

React

const MemoizedButton = [1](Button);

export default [2];

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

Amemo

BButton

CMemoizedButton

DuseMemo

Attempts:

3 left

💡 Hint

Common Mistakes

Exporting the original component instead of the memoized one.

Using useMemo instead of memo for components.

memo is a higher-order component that memoizes the wrapped component. The exported component should be the memoized one to avoid unnecessary renders.

5fill in blank

hard

Fill all three blanks to optimize a list rendering by memoizing the item renderer and using keys correctly.

React

const renderItem = [1]((item) => {
  return <li key={item.[2]>{item.[3]</li>;
}, [items]);

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

AuseCallback

Bid

Cname

DuseMemo

Attempts:

3 left

💡 Hint

Common Mistakes

Using useMemo instead of useCallback for functions.

Using non-unique values for keys causing rendering issues.

useCallback memoizes the function to avoid recreating it each render. The key prop should be a unique identifier like id. The displayed content is the name property.

Practice

1. What is the main purpose of using React.memo in a React component?

easy

A. To convert a class component into a functional component

B. To force the component to re-render every time the parent renders

C. To prevent re-rendering when the component's props have not changed

D. To automatically update the component's state

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 wrapped component if its props remain the same, improving performance.

3. Final Answer:

   To prevent re-rendering when the component's props have not changed -> Option C

4. Quick Check:

   React.memo skips renders if props unchanged = D [OK]

Hint: React.memo skips render if props stay the same [OK]

Common Mistakes:

• Thinking React.memo forces re-render
• Confusing React.memo with state updates
• Believing it converts component types

2. Which of the following is the correct syntax to memoize a function using useCallback in React?

easy

A. const memoizedFn = useCallback(() => { doSomething(); }, []);

B. const memoizedFn = useCallback(doSomething(), []);

C. const memoizedFn = useCallback(() => doSomething);

D. const memoizedFn = useCallback(() => { doSomething(); });

Solution

1. Step 1: Recall useCallback syntax

   useCallback takes a function and a dependency array, returning a memoized version of the function.

2. Step 2: Check each option's correctness

   const memoizedFn = useCallback(() => { doSomething(); }, []); correctly passes a function and an empty dependency array, memoizing the function properly.

3. Final Answer:

   const memoizedFn = useCallback(() => { doSomething(); }, []); -> Option A

4. Quick Check:

   useCallback needs function and dependencies = C [OK]

Hint: useCallback(fn, deps) memoizes function with dependencies [OK]

Common Mistakes:

• Calling the function inside useCallback instead of passing it
• Omitting the dependency array
• Passing function reference without parentheses incorrectly

3. Consider this React component:

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

function App() {
  const [count, setCount] = React.useState(0);
  const handleClick = () => setCount(count + 1);
  return <MyComponent count={count} onClick={handleClick} />;
}

What will happen when App re-renders?

medium

A. MyComponent will never re-render because it is wrapped in React.memo

B. MyComponent will re-render every time because handleClick is recreated on each render

C. MyComponent will re-render only when count changes, ignoring onClick

D. MyComponent will re-render only when handleClick changes, ignoring count

Solution

1. Step 1: Understand React.memo and props

   React.memo skips re-render if props are shallowly equal. Here, onClick is a new function each render.

2. Step 2: Analyze function recreation

   Since handleClick is recreated on every render, its reference changes, causing MyComponent to re-render.

3. Final Answer:

   MyComponent will re-render every time because handleClick is recreated on each render -> Option B

4. Quick Check:

   New function prop breaks memo = B [OK]

Hint: New function props cause re-render despite React.memo [OK]

Common Mistakes:

• Assuming React.memo ignores function prop changes
• Thinking React.memo prevents all re-renders
• Ignoring function identity in props

4. You have this React code that causes unnecessary re-renders:

const Parent = () => {
  const [count, setCount] = React.useState(0);
  const increment = () => setCount(count + 1);
  return <Child onClick={increment} />;
};

const Child = React.memo(({ onClick }) => {
  console.log('Child rendered');
  return <button onClick={onClick}>Click</button>;
});

How can you fix the unnecessary re-renders of Child?

medium

A. Wrap increment in useCallback with count as dependency

B. Remove React.memo from Child

C. Move increment outside the Parent component

D. Use useMemo to memoize count

Solution

1. Step 1: Identify cause of re-renders

   The increment function is recreated on every render, changing its reference and causing Child to re-render.

2. Step 2: Apply useCallback to memoize function

   Wrapping increment in useCallback with count dependency keeps the same function reference unless count changes.

3. Final Answer:

   Wrap increment in useCallback with count as dependency -> Option A

4. Quick Check:

   Memoize functions with useCallback to avoid re-renders = A [OK]

Hint: Memoize event handlers with useCallback to prevent re-renders [OK]

Common Mistakes:

• Removing React.memo which disables optimization
• Moving function outside loses access to state
• Using useMemo incorrectly for functions

5. You want to optimize a component that receives a large list and a filter function as props. The filter function is defined inline in the parent component. Which approach best avoids unnecessary renders of the child component?

hard

A. Use useMemo to memoize the filtered list inside the child component

B. Define the filter function inline without memoization and rely on React.memo alone

C. Avoid React.memo and always re-render the child component to ensure fresh data

D. Wrap the child component with React.memo and memoize the filter function using useCallback with proper dependencies

Solution

1. Step 1: Understand props causing re-renders

   The inline filter function changes reference every render, causing child re-renders despite React.memo.

2. Step 2: Use React.memo and useCallback together

   Wrapping child with React.memo and memoizing the filter function with useCallback keeps function reference stable, avoiding unnecessary renders.

3. Final Answer:

   Wrap the child component with React.memo and memoize the filter function using useCallback with proper dependencies -> Option D

4. Quick Check:

   Memoize functions and use React.memo to optimize renders = A [OK]

Hint: Memoize functions and wrap child with React.memo for best optimization [OK]

Common Mistakes:

• Relying on React.memo without memoizing functions
• Memoizing filtered list inside child instead of function
• Avoiding React.memo and causing extra renders