polyfillio

polyfillio is a web service and content delivery network that serves JavaScript polyfills to browsers on demand. Its core purpose is to minimize payload by loading only the polyfills a given browser actually needs, rather than delivering a fixed bundle for all environments. The service is part of the polyfill-service project and is maintained as an open-source initiative led by the Financial Times, with contributions from the community. The source code and documentation are available on GitHub.

How it works: A webpage loads a script from polyfill.io, typically via a URL such as https://polyfill.io/v3/polyfill.min.js with a features query parameter. The server determines which polyfills are required for the requesting browser based on the browser’s capabilities and the requested features, and returns a JavaScript file that polyfills only those features. The response is cached by the CDN to improve performance and reduce repeat traffic.

Usage: To load the default set of polyfills suitable for most environments, include in the page: <script src="https://polyfill.io/v3/polyfill.min.js?features=default"></script>. Developers can request specific polyfills by name, for example features=Array.prototype.includes%2CPromise. The service also supports non-minified variants and versioned endpoints for advanced use cases.

Notes: As with any external script delivery, sites should consider reliability, latency, and privacy implications when integrating polyfillio into production.