How to use custom JSX elements ?

Last Updated : 16 Nov, 2023

JSX is an extension of the JavaScript language which is useful to write React applications. JSX consist of tags, attribute, and children. We can also create our own custom elements in React.

A custom element gives us a new HTML tag in the JSX file that we can control through a native browser API.

Prerequisites:

Approach:

Initially, what we have to do is to define and form a custom element. To do so we create a class that extends the HTMLElement class and then give the name of the element with customElements.define(). We have three methods to expand the functionality of this element created using custom. There are two lifecycle-like methods that are useful to us the disconnectedCallBack and the connectedCallback and since this is a class, it comes with a constructor.

Constructor: An instance of the element being created or upgraded. Useful for initializing state, setting up event listeners, or creating Shadow DOM. See the spec for restrictions on what you can do in the constructor.

connectedCallback: The element is inserted into the DOM. Useful for running setup code, such as fetching resources or rendering UI. Generally, you should try to delay work until this time

disconnectedCallback: When the element is removed from the DOM. Useful for running clean-up code.

Steps to Create a React Application

Step 1: Create a React application using the following command:

npx create-react-app example

Step 2: After creating your project folder i.e. example, move to it using the following command:

cd example

Step 3: Create a folder component inside the src folder of reacting project directory and inside the components folder create files Counter.jsx and ImperativeCounter.jsx

Project Structure:

Example: This example implements custom JSX element in the imparativeCounter.jsx file

Javascript

// Filename - index.js
 
import React from "react";
import ReactDOM from "react-dom";
import "./index.css";
import App from "./App";
import "./components/ImperativeCounter";
import reportWebVitals from "./reportWebVitals";
 
ReactDOM.render(
    <React.StrictMode>
        <App />
    </React.StrictMode>,
    document.getElementById("root")
);

Javascript

// Filename - App.js
 
import React from "react";
import Counter from "./components/Counter";
 
function App() {
    return (
        <div className="App">
            <Counter />
        </div>
    );
}
 
export default App;

Javascript

// Filename - components/Counter.jsx
 
import React, { useRef } from "react";
 
function Counter() {
    const counterElement = useRef(null);
 
    const incrementCounters = () => {
        // Increment the imperative counter
        counterElement.current.increment();
    };
 
    const decrementCounters = () => {
        // Decrement the imperative counter
        counterElement.current.decrement();
    };
 
    return (
        <div>
            <div>
                <h5>Imperative Counter</h5>
                <i-counter ref={counterElement}></i-counter>
            </div>
            <button
                onClick={incrementCounters}
                className="btn btn-primary"
            >
                Increment
            </button>
            <button
                onClick={decrementCounters}
                className="btn btn-primary"
            >
                Decrement
            </button>
        </div>
    );
}
 
export default Counter;

Javascript

// Filename - components/ImperativeCounter.jsx
 
class ImperativeCounter extends HTMLElement {
    constructor() {
        super();
        this.shadow = this.attachShadow({ mode: "open" });
        this.currentCount = 0;
        this.update();
    }
 
    update() {
        const template = `
        <style>
          .counter {
            font-size: 25px;
          }
        </style>
        <div class="counter">
          <b>Count:</b> ${this.currentCount}
        </div>
      `;
        this.shadow.innerHTML = template;
    }
 
    increment() {
        this.currentCount++;
        this.update();
    }
 
    decrement() {
        this.currentCount--;
        this.update();
    }
}
 
window.customElements.define(
    "i-counter",
    ImperativeCounter
);

Step to Run Application: Run the application using the following command from the root directory of the project:

npm start

Output: This output will be visible on the http://localhost:3000/ on the browser window.

Output

There we have created a custom element that manages its own state and reflects that state back to the user while giving us an HTML element to use in our JSX.

Suggest improvement

How to avoid the need for binding in ReactJS ?

How to return the currently active elements in the HTML document ?

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