React Functional Components

Master React functional components, the building blocks for modern, reusable React UIs. Leverage their simplicity and power to create dynamic and interactive web applications. With React Hooks, functional components can now manage state and side effects, making them a versatile and efficient choice for modern React development.

Table of Contents

React Functional Components

Functional Components were a part of React from the early versions, being present and usable for simple components that did not require state or lifecycle methods from the start. However, the significant enhancement to Functional Components came with the introduction of Hooks in React 16.8, released in February 2019. Hooks allowed Functional Components to use state and other React features previously only available in Class Components. This update made Functional Components much more powerful and versatile.

React Functional Components and Their Role in React Development:

Historical Importance of Functional Components

  • Simpler Syntax
    Historically, React development primarily focused on class-based components. Functional components offered a simpler, less verbose way to define components that only focus on rendering UI.
  • Promoting Reusability
    Functional components are often easier to reuse and compose, focusing solely on UI rendering logic.
  • Separation of Concerns
    Functional components helped solidify the concept of separating rendering logic from state management and complex behaviors.

Alternative to Functional Components

  • Class-based Components: The traditional way of creating React components involved defining classes that extend React.Component. These components can manage state and utilize lifecycle methods.

Why Functional Components Are Relevant

  • Performance Improvements
    Functional components can sometimes offer slight performance improvements as they avoid some overhead that can come with class-based components.
  • Readability
    The simpler syntax of functional components often makes them easier to read and reason about, especially for smaller components.
  • React Hooks
    The introduction of React Hooks revolutionized functional components. Hooks allow you to manage state, side effects, and other features previously limited to class-based components.
  • Modern Standard
    Functional components with React Hooks have become the predominant way of creating React components due to their flexibility and ease of use.

What are React Functional Components?

Functional Component in React is a JavaScript function that returns a React element. It takes in some data (called “props”) and uses it to describe what should appear on the screen. Functional components are known for being simple and focused on the visual aspects of your application. With the help of React Hooks, they can now handle more complex tasks, making them a powerful way to build modern React components.

Key Characteristics

  • Simpler Syntax: Functional components usually have a simpler, more concise syntax than class-based components.
  • Input: Props They receive data as input in the form of an object called props.
  • Output: JSX They return JSX (React’s syntax for describing UI) that determines what will be rendered on the screen.
  • No State or Lifecycle Methods (initially): Traditionally, functional components were stateless, meaning they couldn’t manage their internal data or use lifecycle methods. However, React Hooks changed this.

Modern Functional Components with Hooks:

  • React Hooks: Hooks (like useStateuseEffect) provide a way for functional components to manage state, perform side effects, and have the same capabilities as class-based components.

Creating a Functional Component

Breakdown of how to create a React component:

1. Import React

In a Function Component, you must import React to enable JSX syntax. If you’re using React hooks, you’ll also import those.

import React from 'react';
  • Explanation: We need to import the React library to work with JSX and React components.

2. Define the Functional Component

A Function Component is essentially a JavaScript function that returns a React element. It receives props as its argument.

function Welcome(props) {
  return <h1>Hello, {}!</h1>;
  • Explanation: The Welcome functional component takes an object called props as input. It returns JSX to render a greeting.

Or using arrow function syntax:

const Welcome = (props) => {
  return <h1>Hello, {}</h1>;

3. Functional Component with React Hooks

Functional components cannot use traditional lifecycle methods. Here’s how to use React Hooks (the modern way to manage state and effects in functional components):

import React, { useState, useEffect } from 'react';

function Welcome(props) {
  // Initialize state with a state variable 'count' and setter 'setCount'
  const [count, setCount] = useState(0);

  // Effect hook to update the document title based on 'count' state changes
  useEffect(() => {
    document.title = `You clicked ${count} times`;

  // Component return with JSX
  • Explanation:
    • useState: This hook introduces state into the component (count variable).
    • useEffect: This hook lets you perform side effects (like updating the document title).

4. Render Method

Function Components don’t have a render method. They return JSX directly from the body of the function.

5. Using a Functional Component

React allows you to use Functional and Class Components within the same application. Functional Components are preferred for their simplicity and use of hooks for state and lifecycle features. However, Class Components are still fully supported and may be used when preferred or necessary.

<Welcome name="Alice" />


When React encounters Welcome Component:

  • Creates a props object that looks like { name: 'Alice' }.
  • Calls the Welcome function with the props object.
  • The Welcome function returns a React element (JSX), which describes what should appear on the screen.
  • React then updates the DOM to match the output of the Welcome component.

Complete Example: Greeting Component

// In Greetings.jsx
import React from 'react';  // Import React library at the beginning

function Greeting(props) {  // Defines the component, 'props' is a prop
    return (
        <h1>Hello, {}!</h1>  // JSX for content


  • Line 2: Imports the React library.
  • Line 4-8: Defines a functional component named Greeting that takes props as its argument.
    • props allow you to pass data to your component.
  • Line 5-7: The return statement defines the JSX content for the component.
    • Line 6: Uses JSX to return an <h1> HTML tag displaying a greeting message that includes the name prop.
      • It utilizes curly braces { } to embed the name prop value within the text.

Now that you’ve defined your component’s functionality, it’s time to make it usable within your React application. This involves exporting the component to be referenced and used from other parts of your code.

Exporting the Greeting Component

In JavaScript, the export keyword allows you to share functionalities with other files. Here’s how it applies to React components:

export function Greeting(props) {  // exports the component
    return (
        <h1>Hello, {}!</h1>  // JSX for content


  • Line 1: We’ve added the export keyword before the function keyword. This tells JavaScript to make this function accessible from outside the current file.

Remember: Without exporting, your component remains hidden within the file where it’s defined and cannot be used elsewhere in your application.

Function Components offer a simpler and more concise way to create components in React, especially with the introduction of hooks, which allow functional components to fully embrace React’s features like state and lifecycle methods without the need for classes.

Rendering Your Components

Once you’ve defined and exported your components, it’s time to render them on the screen. React uses a special library called ReactDOM to handle this process.


  • You’ll provide a reference to your component and a target HTML element where you want it displayed.
  • React efficiently updates the UI based on your component structure and any changes that occur.

Example: Rendering the Greeting Component

Assuming you have a Greeting component defined and exported (from previous steps), here’s how to render it:

// In the file where App component is defined (e.g., App.jsx)
import React from 'react';  // Import React library
import ReactDOM from 'react-dom'; // Import ReactDOM for rendering
import { Greeting } from './Greetings';  // Import the Greeting component correctly

function App() {
    return (
        <div>  {/* Target element (a DOM element) */}
            <Greeting name="Alice" />  {/* Render the Greeting component with props */}

// Assuming you're working within an environment where ReactDOM.createRoot is valid
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<App />);  // Render the App component


  • Lines 2-4: Import statements:
    • Line 2: Imports the React library, which is necessary for using JSX and React components.
    • Line 3: Imports the ReactDOM library for rendering React components to the DOM.
    • Line 4: Imports the Greeting component from the Greetings.jsx file, making it usable in this file.
  • Lines 6-12: The App component:
    • This component defines a JSX structure representing the UI you want to render.
    • Line 8: A div element acts as the target container.
    • Line 9: The Greeting component is used within the div. Notice it receives a name prop set to "Alice".
  • Lines 15-16: Rendering:
    • ReactDOM.createRoot creates a root element for React to manage.
    • root.render(<App />) tells React to render the App component within the root element (referencing the div with id "root" in the HTML).

Following this pattern, you can render your custom components within your React application, allowing you to build user interfaces with reusable building blocks.

Understanding Props in React

Think of props in React as packages sent from parent components to child components. Props contain information, like names, numbers, or other components, that help customize how the child component looks or behaves. Child components receive and use these props, but they can’t change the contents of the package itself. This ensures that data flows predictably through your app. Props are key to creating reusable and adaptable components in React, making your code cleaner and easier to manage.


React components are powerful because they can be reused and customized. This is where props come into play. Props act as arguments passed into functions, but in the context of React, they are a way for parent components to provide data or configuration to their child components.

Understanding Props

  • Props are like data packets containing information that child components can use.
  • They allow parent components to control the behavior and appearance of their child components.
  • Child components receive props as arguments within their function definition.

Example: Button Label Prop

// Button component (assuming it displays a label)
function Button({label}) {
    return (
        <button>{label}</button>  /* JSX for content, uses the 'label' prop */


  • Line 2: This line defines a component named Button using a function. It takes a single argument named label. This label argument is the prop.
  • Line 4: The JSX content for the button displays the value of the label prop within the button text.

Using props, you can create versatile components that adapt their behavior or appearance based on the data they receive from parent components.

Props: One-Way

One important concept to remember about props in React is that they are read-only for child components. This means that a child component cannot directly modify the value of a prop it receives from a parent component.

Why Read-Only Props?

  • It promotes predictable behavior: Since props are fixed values, child components behave consistently based on the data.
  • Encourages unidirectional data flow: Changes in data typically flow from parent to child, making the application easier to reason about and debug.


import React from 'react';
import ChildComponent from './ChildComponent';

function ParentComponent() {
    const message = "Hello from Parent!";

    return (
            <ChildComponent message={message} />


function ChildComponent(props) {
    return <h1>{props.message}</h1>;


  • ParentComponent.jsx
    • Line 1: Imports the React library to use JSX and component functionality.
    • Line 2: Imports the ChildComponent from its file for use within this component.
    • Line 4: Defines the ParentComponent, which is a functional component.
    • Line 5: Declares a constant message that holds “Hello from Parent!”.
    • Line 9: Returns JSX content with an <h1> element displaying the message prop.
      • The ChildComponent is used here with a prop named message. This demonstrates the one-way data flow from parent to child component.
  • ChildComponent.jsx
    • Line 1: Defines the ChildComponent with props as its parameter, allowing it to receive props from a parent component.
    • Line 2: Returns an h1 element displaying the message prop. This line shows how props pass data from parent to child components, reinforcing the concept of one-way data flow in React.

Even though the ChildComponent component receives a message prop, it cannot directly change the value of that prop within the component itself. The message prop remains fixed as defined by the ParentComponent.

Composing and Nesting Components

React’s power lies in its ability to create complex UIs by combining simpler components. This is achieved through composition and nesting.


  • Imagine components as building blocks. You can combine these blocks to form more intricate structures.
  • Each component can be responsible for a specific UI part, promoting code reusability and maintainability.


  • Components can contain other components within their JSX structure. This allows for the hierarchical organization of your UI.
  • Child components inherit the props their parent component provides, offering further customization options.

Example: Card Component

// Card component (assuming it displays a title and content)
function Card(props) {
    return (
        <div className="card">
            <h2>{props.title}</h2>  {/* Display title prop */}
            <p>{props.content}</p>  {/* Display content prop */}

// App component (assuming it renders multiple cards)
function App() {
    return (
            <Card title="Card 1" content="This is the content of card 1." />  {/* Use Card component with props */}
            <Card title="Card 2" content="This is the content of card 2." />  {/* Use Card component with props */}


  • Lines 2-9: The Card component:
    • Line 2: Takes props as an argument.
    • Lines 5-6: Uses props (title and content) within JSX to display content in the card.
  • Lines 12-19: The App component:
    • Lines 15-16: Renders two Card components.
    • Each Card component receives props (title and content) to customize its content.

Compelling and nesting components allow you to create complex and flexible UIs by combining reusable building blocks. This approach promotes cleaner code, easier maintenance, and a more modular way of building user interfaces in React applications.

Extracting Components for Reusability

As your React application grows, you might write similar UI elements in multiple places. This can lead to code duplication and make maintenance difficult. Here’s where extracting components comes in.

The Power of Extraction

  • Identify repetitive UI structures within your code.
  • Extract those structures into separate, reusable components.
  • This reduces code duplication and promotes maintainability.

Example: Formatting Text

Imagine you have a Post component that displays a title and some content. The title should be bolded consistently across all posts. Here’s how to extract a component:

Before Extraction (Potential Duplication)

function Post(props) {
    return (
        {/* Bold title (might be duplicated elsewhere) */}

After Extraction (Reusable BoldText Component)

// BoldText component
function BoldText(props) {
    return <b>{props.children}</b>; /* Display content passed as children */

// Post component using BoldText
function Post(props) {
    return (
            <BoldText>{props.title}</BoldText> {/* Use BoldText component with title */}


  • Lines 2-4: BoldText component:
    • The BoldText component uses props.children to display any child elements or text between its opening and closing tags.
    • Takes content as children using curly braces { }.
    • Wraps the children in a <b> element for bold formatting.
  • Lines 7-14: Post component:
    • Line 10: BoldText encloses props.title, treating it as its children. This is how you should pass content to components that are designed to render their children prop.

By extracting the bold formatting logic into a reusable BoldText component, you avoid duplication and can easily apply bold formatting elsewhere in your application. This approach keeps your code cleaner and easier to manage.

Components in Separate Files

As your React project grows, keeping all your components within a single file can become cumbersome. Here’s where separating components into individual files comes into play.

Benefits of Separate Files

  • Improved organization: Separating components makes your codebase more organized and easier to navigate.
  • Maintainability: Changes to a specific component are isolated within its file.
  • Reusability: You can import components from other files, promoting code reuse across your application.

Example: Greeting Component (in Separate File)


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

export default Greeting;  // Export the component

App.jsx (assuming it uses the Greeting component)

import Greeting from './greetings';  // Import the Greeting component

function App() {
    return (
            <Greeting name="Alice" />

export default App;  // Assuming App is the main component


  • greetings.jsx (Lines 1-7):
    • This file defines the Greeting component.
    • Line 7: Exports the component using export default Greeting.
  • App.jsx (Lines 1-9):
    • Line 1: Imports the Greeting component from the ./greetings file (assuming the file is in the same directory).
    • Lines 5-7: Uses the imported Greeting component within the JSX of the App component.

Separating components into files improves code organization, maintainability and reusability across your React application.

What the Browser Sees (JSX Transpilation)

When you write React components using JSX, it might seem like the browser directly understands this syntax. However, that’s not quite true. JSX is a special syntax extension for React that gets converted into regular JavaScript before the browser sees it.

The Transpilation Process

  • JSX code is written within your React components.
  • A tool like Babel (a JavaScript compiler) is used during development to convert JSX into plain JavaScript function calls.
  • The converted JavaScript code is then what the browser understands and executes.

Example: JSX to JavaScript Conversion

Before Transpilation (JSX)

function Button({label}) {
    return (


  • Lines 1-5: This code defines a Button component using JSX for the button element.

After Transpilation (JavaScript)

function Button(props) {
    return React.createElement('button', null, props.label);


  • Line 1: Same function definition for Button.
  • Line 2: The JSX is converted to a call to React.createElement. React provides this function and takes three arguments:
    • The first argument ("button") specifies the HTML element type (button in this case).
    • The second argument ({ }) is an empty object (can be used for attributes).
    • The third argument (props.label) is the content for the button.

By transpiling JSX, the browser receives regular JavaScript code to understand and execute. This allows React to manipulate the DOM and render your UI as intended.