Reusable components in React

Introduction

Reusable components let you build parts of your app once and use them many times. This saves time and keeps your code neat.

When you have a button style used in many places.

When you want to show a user profile card in different screens.

When you need a form input that behaves the same everywhere.

When you want to keep your app organized by breaking UI into small pieces.

When you want to easily update a feature in one place and see changes everywhere.

Syntax

React

function ComponentName(props) {
  return (
    <div>{props.content}</div>
  );
}

// Usage
<ComponentName content="Hello!" />

Components are functions that return UI elements.

Props are inputs you give to components to customize them.

Examples

A simple button component that shows text from props.

React

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

<Button label="Click me" />

Using destructuring to get props directly for cleaner code.

React

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

<Greeting name="Alice" />

Component with children lets you put any content inside it.

React

function Card({ title, children }) {
  return (
    <div className="card">
      <h2>{title}</h2>
      <div>{children}</div>
    </div>
  );
}

<Card title="My Card">
  <p>This is inside the card.</p>
</Card>

Sample Program

This app shows two buttons using the same reusable Button component. Each button has its own label and click action. The buttons are accessible with aria-label and keyboard friendly.

React

import React from 'react';

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

export default function App() {
  const handleClick = () => alert('Button clicked!');

  return (
    <main>
      <h1>Reusable Button Example</h1>
      <Button label="Say Hello" onClick={handleClick} />
      <Button label="Say Goodbye" onClick={() => alert('Goodbye!')} />
    </main>
  );
}

Important Notes

Always give components clear names that describe what they do.

Use props to make components flexible and reusable in different places.

Remember to add accessibility features like aria-label for better user experience.

Summary

Reusable components help you write less code and keep your app organized.

They take inputs called props to change how they look or behave.

You can use the same component many times with different props.

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