Understanding Polyfills in React

What is a Polyfill?

A polyfill is a piece of code that mimics the behavior of a newer JavaScript feature, allowing it to work on older browsers that don’t support it natively. It’s essentially a fallback that fills the gap between the browser’s capabilities and the feature you want to use.

Using Polyfills in React

To demonstrate how polyfills work, let’s create a simple React application that uses the String.prototype.padEnd feature, which is not supported by older browsers.

The Feature to be Implemented

We want to display a list of weekdays, each followed by an emoji, with equal spacing between the day and the emoji. We’ll use the padEnd feature to achieve this.

Project Setup

Clone the starter React and Vite template from here. Install the required dependencies and start the application.

Simulating Older Browser Versions

To simulate older browser versions, we’ll add a script to the index.html file that deletes the padEnd feature from the String.prototype.

<script>
  delete String.prototype.padEnd;
</script>

Writing a Polyfill from Scratch

Let’s write our own polyfill for the padEnd feature. We’ll add a check to see if the feature is supported, and if not, provide a fallback implementation.

if (!String.prototype.padEnd) {
  String.prototype.padEnd = function(targetLength, padString) {
    // implementation
  };
}

Using a Polyfill Library

Instead of writing our own polyfill, we can use a library like Polyfill.io or core-js to provide the fallback implementation.

Loading from a CDN

We can include the polyfill library from a CDN using a script tag in our index.html file.

<script src="https://example.com/polyfill.min.js"></script>

Loading from an npm Package

Alternatively, we can install the polyfill library as an npm package and import it in our React component.

npm install core-js

import 'core-js/modules/es.string.pad-end';

• Writing our own polyfill from scratch
• Loading from a CDN
• Loading from an npm package

By understanding how polyfills work, we can ensure our applications work seamlessly across different browsers and versions.