React Fundamental: Custom components

🌟 Understanding Custom Components in React: A Beginner-Friendly Guide

One of the most powerful features of React is the ability to create custom components. They make your code reusable, more readable, and easier to maintain. If you’re already familiar with JavaScript functions, you’re halfway there. In this post, we’ll explore how React components work and how to write them using both the raw API and JSX.

📌 What Is a Component in React?

In React, components are just functions.

Yes, you read that right!

A React component is a function that accepts an object called props and returns something that can be rendered.

Here’s a simple example:

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

You can use this component just like a regular HTML tag in JSX:

<Greeting name="World" />

🧱 Building Simple Components with Functions

Before jumping fully into React’s component model, let’s start with a basic function that returns a React element. For instance, say you want to create the following HTML structure:

<div class="container">
    <div class="message">Hello World</div>
    <div class="message">Goodbye World</div>
</div>

You can define a message function like this:

function message(children) {
    return <div className="message">{children}</div>
}

Then use it in JSX like so:

<div className="container">
    {message('Hello World')}
    {message('Goodbye World')}
</div>

This isn’t the standard way to define React components, but it helps illustrate how functions can return JSX.

🛠️ Introducing React’s Raw API

React’s createElement function is what JSX gets compiled into. Normally, you might see:

React.createElement('div', { className: 'message' }, 'Hello World')

But you can also pass a function as the first argument:

function Message(props) {
    return <div className="message">{props.children}</div>
}

const element = React.createElement(Message, { children: 'Hello World' })

React will call Message with the provided props, and use the JSX it returns.

💡 Using JSX to Write Custom Components

While using createElement is powerful, JSX is way more intuitive and readable. Let’s take the same example and write it using JSX:

function Message(props) {
    return <div className="message">{props.children}</div>
}

const element = <Message>Hello World</Message>

Much cleaner, right?

🧠 But wait — why does this work?

JSX elements are transformed by Babel into React.createElement calls. But there’s a rule:

The name of the component must be capitalized for Babel to treat it as a custom component.

If you write <message>Hello</message>, Babel will interpret message as a string and generate:

React.createElement('message', {}, 'Hello')

But if you use <Message>, it becomes:

React.createElement(Message, {}, 'Hello')

So always capitalize your component names!

🧪 JSX Naming Conventions Breakdown

Here’s how Babel interprets JSX based on naming:

Avoid lowercase or kebab-case names for custom components, or Babel will think you’re creating a standard DOM element!

🧮 Practicing with Props: A Calculator Component

Let’s now build a more dynamic component: a Calculator that receives props and renders an expression with its solution.

function Calculator(props) {
    const { left, operator, right } = props
    let result

    switch (operator) {
        case '+':
            result = left + right
            break
        case '-':
            result = left - right
            break
        case '*':
            result = left * right
            break
        case '/':
            result = left / right
            break
        default:
            result = 'Invalid operator'
    }

    return (
        <div>
            <code>
                {left} {operator} {right} = <output>{result}</output>
            </code>
        </div>
    )
}

Use it like this:

<Calculator left={1} operator="+" right={2} />

This would render:

<div>
  <code>
    1 + 2 = <output>3</output>
  </code>
</div>

🧠 Summary

React components are just functions that return something renderable. To make them work seamlessly with JSX:

✅ Capitalize your component names ✅ Pass props as the function argument ✅ Return valid JSX (or null, strings, numbers, etc.) ✅ Embrace reusability and readability

React’s magic isn’t so mysterious when you peel back the layers — it’s just functions, props, and return values. And thanks to JSX and Babel, we get to write expressive, intuitive code that compiles down to performant raw APIs.

📚 Further Reading

• JavaScript in JSX with Curly Braces – React Docs
• What is JSX? – Kent C. Dodds

Happy Reacting! ⚛️💻

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