What are JavaScript polyfills for?

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What Are JavaScript Polyfills?

A polyfill is a piece of code (usually written in JavaScript) that provides modern functionality to older browsers or environments that do not natively support certain features.

Polyfills are used to "fill in" gaps in browser compatibility by emulating missing features so that your code can run consistently across different environments.

Why Are Polyfills Needed?

JavaScript evolves over time, introducing new features in ES6 (ECMAScript 2015) and later versions.
Older browsers or environments may not support these features.
Polyfills allow developers to use modern JavaScript features without worrying about breaking functionality in older browsers.

How Do Polyfills Work?

A polyfill checks whether a certain feature exists in the browser.
If the feature does not exist, it implements the feature using older JavaScript that is supported.

For example, if the Array.prototype.includes() method is missing in an older browser, a polyfill can add it manually.

Example: Polyfill for `Array.prototype.includes()`

The includes() method checks whether an array contains a certain value. It was introduced in ES6, but older browsers like Internet Explorer do not support it.

Here’s a polyfill for includes():

if (!Array.prototype.includes) {
  Array.prototype.includes = function (searchElement, fromIndex) {
    if (this == null) throw new TypeError('"this" is null or not defined');
    let array = Object(this);
    let length = array.length >>> 0;
    if (length === 0) return false;

    let startIndex = fromIndex || 0;
    let k = Math.max(
      startIndex >= 0 ? startIndex : length - Math.abs(startIndex),
      0
    );

    while (k < length) {
      if (
        array[k] === searchElement ||
        (Number.isNaN(array[k]) && Number.isNaN(searchElement))
      ) {
        return true;
      }
      k++;
    }
    return false;
  };
}

The code first checks if includes() already exists.
If it doesn't, the polyfill manually implements the same logic.

Common Use Cases for Polyfills

New JavaScript Methods or Features

Example: Array.prototype.includes, Object.assign, Promise, fetch API.

Browser-Specific Features

Ensuring features like requestAnimationFrame, localStorage, or JSON are available in older browsers.

HTML5 and CSS3 Features

Some polyfills can emulate HTML5 features like the <canvas> element or provide CSS support for things like Flexbox.

How Are Polyfills Used?

Manually Adding Polyfills: You can add a polyfill script directly to your project, often before your main scripts.
Using Libraries: Popular libraries like Babel and core-js automatically include polyfills for modern JavaScript features.
CDNs: Services like polyfill.io provide tailored polyfills based on the browser's user agent.

Example of Loading Polyfills via CDN:

<script src="https://polyfill.io/v3/polyfill.min.js"></script>

Popular Polyfill Libraries

core-js: A library for ES6+ polyfills (used by Babel).
polyfill.io: A service that serves polyfills specific to the browser requesting the page.
es6-promise: Provides a polyfill for the ES6 Promise API.

Summary

A polyfill is a piece of code that provides support for modern JavaScript features in environments or browsers that do not natively support them.
Polyfills enable consistent behavior across different platforms and browsers.
They are essential for developers who want to use modern features while ensuring compatibility with older browsers.

MDN Web Docs: Polyfills

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What are JavaScript polyfills for?

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