Functional components in React

Introduction

Functional components let you create parts of a webpage using simple JavaScript functions. They make building user interfaces easy and clear.

When you want to create a simple UI part that shows some content.

When you need a reusable piece of UI that can accept data and display it.

When you want to use React hooks to add state or side effects.

When you want your code to be easy to read and maintain.

When you want to build modern React apps without using classes.

Syntax

React

function ComponentName(props) {
  return (
    <div>
      {/* UI elements here */}
    </div>
  );
}

The function name should start with a capital letter.

It returns JSX, which looks like HTML but is JavaScript.

Examples

A simple component that shows a greeting message.

React

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

This component uses a prop to show a personalized welcome.

React

function Welcome(props) {
  return <h2>Welcome, {props.name}!</h2>;
}

A functional component using a hook to add state and update the UI.

React

import React, { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);
  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Add 1</button>
    </div>
  );
}

Sample Program

This component shows how to track button clicks using state in a functional component. It updates the number each time you click the button.

React

import React, { useState } from 'react';

function ClickTracker() {
  const [clicks, setClicks] = useState(0);

  return (
    <main>
      <h1>Click Tracker</h1>
      <p>You clicked {clicks} times.</p>
      <button onClick={() => setClicks(clicks + 1)} aria-label="Add one click">Click me</button>
    </main>
  );
}

export default ClickTracker;

Important Notes

Always start component names with a capital letter to help React recognize them.

Use hooks like useState to add state inside functional components.

Functional components are easier to test and reuse than class components.

Summary

Functional components are simple JavaScript functions that return UI.

They can accept data through props and use hooks for state and effects.

They are the modern, recommended way to build React components.

Practice

1. What is a React functional component?

easy

A. A JavaScript function that returns JSX to render UI

B. A class that extends React.Component

C. A CSS style sheet for React components

D. A special HTML tag used in React

Solution

1. Step 1: Understand React component types

   React components can be functions or classes that return UI elements.

2. Step 2: Identify functional component definition

   A functional component is a plain JavaScript function returning JSX, not a class or style.

3. Final Answer:

   A JavaScript function that returns JSX to render UI -> Option A

4. Quick Check:

   Functional component = function returning JSX [OK]

Hint: Functional components are just functions returning JSX [OK]

Common Mistakes:

• Confusing functional components with class components
• Thinking functional components are CSS or HTML tags
• Believing functional components must extend React.Component

2. Which of the following is the correct syntax for a React functional component named Greeting?

easy

A. const Greeting = Hello!;

B. function Greeting() { return Hello!; }

C. class Greeting extends React.Component { render() { return Hello!; } }

D. function Greeting { return Hello!; }

Solution

1. Step 1: Check function declaration syntax

   function Greeting() { return

   Hello!

   ; } correctly declares a function with parentheses and returns JSX.

2. Step 2: Identify syntax errors in other options

   const Greeting =

   Hello!

   ; assigns JSX directly without a function wrapper; class Greeting extends React.Component { render() { return

   Hello!

   ; } } is a class component, not functional; function Greeting { return

   Hello!

   ; } misses parentheses in function declaration.

3. Final Answer:

   function Greeting() { return

   Hello!

   ; } -> Option B

4. Quick Check:

   Function syntax with parentheses and return JSX [OK]

Hint: Functional components need parentheses after function name [OK]

Common Mistakes:

• Omitting parentheses in function declaration
• Confusing class components with functional components
• Assigning JSX directly without a function

3. What will be rendered by this React functional component?

function Welcome(props) {
  return <h2>Hello, {props.name}!</h2>;
}

// Usage: <Welcome name="Alice" />

medium

A. <Welcome name="Alice" />

B. Hello, {props.name}!

C. Error: props is undefined

D. <h2>Hello, Alice!</h2>

Solution

1. Step 1: Understand props usage in functional components

   The component receives props and uses props.name inside JSX to display the name.

2. Step 2: Substitute props.name with passed value

   Since name="Alice" is passed, {props.name} becomes "Alice" in the output.

3. Final Answer:

   <h2>Hello, Alice!</h2> -> Option D

4. Quick Check:

   props.name replaced by "Alice" in JSX output [OK]

Hint: Props values replace placeholders inside JSX [OK]

Common Mistakes:

• Thinking JSX shows literal {props.name} text
• Assuming props is undefined without passing
• Confusing component usage with rendered output

4. Identify the error in this React functional component:

function Counter() {
  const [count, setCount] = useState(0);
  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

medium

A. Missing return statement

B. Incorrect JSX syntax inside return

C. useState is not imported from React

D. setCount should not be called inside onClick

Solution

1. Step 1: Check useState usage

   useState is a React hook and must be imported explicitly from 'react'.

2. Step 2: Verify other code parts

   The return statement and JSX syntax are correct; calling setCount in onClick is valid.

3. Final Answer:

   useState is not imported from React -> Option C

4. Quick Check:

   Hooks require import from React [OK]

Hint: Always import useState from React before using it [OK]

Common Mistakes:

• Forgetting to import useState
• Thinking setCount can't be called in event handlers
• Assuming JSX syntax errors without checking imports

5. You want to create a functional component that shows a list of items passed as props and highlights the selected item on click. Which approach correctly uses hooks and props together?

hard

A. Use useState to track selected item and map props.items to render list with onClick handlers updating state

B. Use class component with this.state and this.props to manage selected item and render list

C. Use useEffect to directly modify props.items and highlight selected item

D. Render props.items without state and rely on CSS :hover to highlight selection

Solution

1. Step 1: Understand state and props roles

   Props provide data (items), state tracks UI changes (selected item).

2. Step 2: Choose correct React hooks usage

   useState is used to track selected item; mapping props.items with onClick updates state correctly.

3. Step 3: Evaluate other options

   Class components are outdated here; useEffect should not modify props; CSS :hover can't track selection state.

4. Final Answer:

   Use useState to track selected item and map props.items to render list with onClick handlers updating state -> Option A

5. Quick Check:

   State for selection + props for data = Use useState to track selected item and map props.items to render list with onClick handlers updating state [OK]

Hint: Use useState for UI changes, props for data input [OK]

Common Mistakes:

• Trying to modify props directly
• Using class components instead of functional with hooks
• Relying on CSS hover for persistent selection