Reusable components in React - Performance & Optimization

Performance: Reusable components

MEDIUM IMPACT

Reusable components affect rendering speed and bundle size by reducing duplicated code and minimizing unnecessary DOM updates.

Creating UI elements that appear multiple times with similar structure

React

function Button({ label, onClick }) {
  return <button onClick={onClick}>{label}</button>;
}

function App() {
  return (
    <>
      <Button label="Click 1" onClick={() => alert('Clicked 1')} />
      <Button label="Click 2" onClick={() => alert('Clicked 2')} />
      <Button label="Click 3" onClick={() => alert('Clicked 3')} />
    </>
  );
}

Reusing the Button component reduces code duplication and allows React to optimize rendering and updates.

📈 Performance GainSmaller bundle size and fewer unnecessary re-renders, improving interaction responsiveness

Creating UI elements that appear multiple times with similar structure

React

function App() {
  return (
    <>
      <button onClick={() => alert('Clicked 1')}>Click 1</button>
      <button onClick={() => alert('Clicked 2')}>Click 2</button>
      <button onClick={() => alert('Clicked 3')}>Click 3</button>
    </>
  );
}

Duplicated button code increases bundle size and makes maintenance harder; React treats each button as a separate element causing more DOM nodes.

📉 Performance CostAdds unnecessary bundle size and triggers multiple reflows if buttons update independently

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Duplicated inline elements	Many unique nodes	Multiple reflows on updates	Higher paint cost due to many nodes	[X] Bad
Reusable components	Fewer unique nodes, reused	Single or fewer reflows	Lower paint cost due to reuse	[OK] Good

Rendering Pipeline

Reusable components reduce the amount of unique code React must process and help React reuse DOM nodes efficiently during reconciliation.

→JavaScript Execution

→Virtual DOM Diffing

→DOM Updates

⚠️ BottleneckExcessive unique elements increase Virtual DOM diffing and DOM updates

Core Web Vital Affected

INP

Reusable components affect rendering speed and bundle size by reducing duplicated code and minimizing unnecessary DOM updates.

Optimization Tips

1Use reusable components to avoid repeating similar JSX code.

2Keep components small and focused to maximize reuse and minimize re-renders.

3Pass only necessary props to reusable components to avoid unnecessary updates.

Performance Quiz - 3 Questions

How do reusable components improve React app performance?

A By increasing the number of DOM nodes to speed up rendering

B By reducing duplicated code and minimizing unnecessary re-renders

C By forcing React to reload the entire page on each update

D By disabling React's virtual DOM diffing

DevTools: React DevTools and Chrome Performance

How to check: Use React DevTools to inspect component tree and check for repeated code. Use Performance panel to record interactions and see re-render counts.

What to look for: Look for repeated components in the tree and excessive re-renders causing long scripting times or layout shifts.

Practice

1. What is the main benefit of using reusable components in React?

easy

A. They make your app run faster by default

B. They let you write less code and keep your app organized

C. They automatically fix bugs in your code

D. They prevent users from clicking buttons

Solution

1. Step 1: Understand the purpose of reusable components

   Reusable components are designed to be used multiple times to avoid repeating code.

2. Step 2: Identify the main benefit

   Using reusable components helps keep the app organized and reduces the amount of code you write.

3. Final Answer:

   They let you write less code and keep your app organized -> Option B

4. Quick Check:

   Reusable components = less code + organized app [OK]

Hint: Reusable means use again to save code and stay organized [OK]

Common Mistakes:

• Thinking reusable components speed up the app automatically
• Believing they fix bugs without coding
• Confusing reusable components with user interaction controls

2. Which of the following is the correct way to pass a prop named title with value "Hello" to a reusable component Card?

easy

A. <Card title=Hello />

B. <Card title={Hello} />

C. <Card "Hello"=title />

D. <Card title="Hello" />

Solution

1. Step 1: Recall JSX prop syntax

   Props are passed as attributes with name and value inside quotes or curly braces for expressions.

2. Step 2: Check each option

   <Card title="Hello" /> uses correct syntax: title="Hello". <Card title={Hello} /> misses quotes around string. <Card title=Hello /> lacks quotes or braces. <Card "Hello"=title /> has incorrect attribute format.

3. Final Answer:

   <Card title="Hello" /> -> Option D

4. Quick Check:

   Props need name="value" syntax [OK]

Hint: Props look like HTML attributes with quotes [OK]

Common Mistakes:

• Forgetting quotes around string prop values
• Using curly braces without variables
• Swapping prop name and value order

3. What will be the output of this React component usage?

function Greeting(props) {
  return <h1>Hello, {props.name}!</h1>;
}

<Greeting name="Alice" />
<Greeting name="Bob" />

medium

A. Hello, Alice!Hello, Bob!

B. Hello, Alice!

C. Hello, Bob!

D. Error: Cannot render two components like this

Solution

1. Step 1: Understand component usage

   Each <Greeting /> renders an <h1> with the passed name prop.

2. Step 2: Check the two usages

   First renders Hello, Alice!, second renders Hello, Bob!. Both outputs appear in order.

3. Final Answer:

   <h1>Hello, Alice!</h1><h1>Hello, Bob!</h1> -> Option A

4. Quick Check:

   Two components with different props render two greetings [OK]

Hint: Each component instance renders separately with its props [OK]

Common Mistakes:

• Thinking only one component renders
• Believing React errors on multiple components
• Confusing props with state causing wrong output

4. Identify the error in this reusable component code:

function Button(props) {
  return <button>Click {props.label}</button>
}

// Usage
<Button label=Submit />

medium

A. Button component must return a div, not button

B. Props cannot be used inside JSX

C. Missing quotes around the prop value in usage

D. Component name must start with lowercase

Solution

1. Step 1: Check prop usage syntax

   In JSX, string prop values must be in quotes or curly braces.

2. Step 2: Identify the error

   The usage <Button label=Submit /> lacks quotes around "Submit" causing syntax error.

3. Final Answer:

   Missing quotes around the prop value in usage -> Option C

4. Quick Check:

   String props need quotes [OK]

Hint: String props always need quotes in JSX [OK]

Common Mistakes:

• Thinking button must be wrapped in div
• Believing props can't be used inside JSX
• Using lowercase component names

5. You want to create a reusable List component that shows items with a custom renderItem prop function. Which approach correctly uses renderItem inside the component?

hard

A. function List(props) {
return <ul>{
props.items.map(item => <li>{props.renderItem(item)}</li>)
}</ul>;
}

B. function List(props) {
return <ul>{
props.items.map(props.renderItem(item))
}</ul>;
}

C. function List(props) {
return <ul>{
props.renderItem(props.items)
}</ul>;
}

D. function List(props) {
return <ul>{
props.items.forEach(item => props.renderItem(item))
}</ul>;
}

Solution

1. Step 1: Understand how to use renderItem with map

   To render a list, use map to transform each item into JSX, calling renderItem for each.

2. Step 2: Check each option

   function List(props) {
     return <ul>{
       props.items.map(item => <li>{props.renderItem(item)}</li>)
     }</ul>;
   }

   correctly maps items and calls renderItem inside <li>.

   function List(props) {
     return <ul>{
       props.items.map(props.renderItem(item))
     }</ul>;
   }

   calls renderItem incorrectly inside map.

   function List(props) {
     return <ul>{
       props.renderItem(props.items)
     }</ul>;
   }

   calls renderItem once with whole array.

   function List(props) {
     return <ul>{
       props.items.forEach(item => props.renderItem(item))
     }</ul>;
   }

   uses forEach which returns undefined, so no output.

3. Final Answer:

   Option A uses map and renderItem correctly inside list -> Option A

4. Quick Check:

   Use map with renderItem for lists [OK]

Hint: Use map with renderItem inside JSX to render lists [OK]

Common Mistakes:

• Using forEach instead of map for rendering
• Calling renderItem outside map or with wrong argument
• Passing whole array to renderItem instead of single item