polyfill-service

Polyfill.io is a web service that helps websites work correctly across a wide range of browsers, including older ones that are missing support for newer web features. When a browser loads a webpage and requests this service, Polyfill.io reads information about which browser is making the request and sends back only the code snippets that browser actually needs, nothing more. This matters because different browsers support different things, and keeping a website functional for all of them can otherwise require a lot of manual, error-prone work by the developer.

The core idea behind a polyfill is to fill in a missing feature. If a browser does not support a particular modern JavaScript capability, a polyfill adds a version of that capability to the browser for that page visit. Without a service like this, developers have to either include a large bundle of compatibility code for every visitor (most of which goes to waste on modern browsers) or test and target each browser individually. Polyfill.io handles this automatically by inspecting the incoming request and building a tailored response.

The hosted version at polyfill.io is free and now runs through Cloudflare as its delivery network, after a migration the team announced in the README. For teams that prefer to run this infrastructure themselves rather than rely on an external service, a self-hosted version is available in a separate repository.

The project is open source under the MIT license. Its documentation lives on the polyfill.io website rather than in the repository itself, so the README is brief and mostly points outward to those external resources. Contributions are accepted via pull request, and the contributing guide in the repository explains the development process and the contribution license agreement that contributors need to accept.