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polyfillpolyfill/polyfill-service

★ 7,153JavaScriptAudience · developerComplexity · 2/5LicenseSetup · easy

TLDR

Polyfill.io detects which browser is loading your page and serves only the missing modern JavaScript features that browser needs, keeping older browsers working without bloating load times for everyone else.

Mindmap

mindmap
  root((polyfill-service))
    What it does
      Browser detection
      Targeted polyfill delivery
    How it works
      Reads User-Agent
      Sends only needed code
    Delivery
      Hosted at polyfill.io
      Self-hosted option
    Audience
      Web developers
      Frontend teams
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Code map

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Things people build with this

USE CASE 1

Add a single script tag so older browsers receive the JavaScript features they need without any manual browser testing.

USE CASE 2

Self-host the polyfill service inside your own infrastructure to avoid depending on a third-party CDN.

USE CASE 3

Reduce the JavaScript payload for modern browser visitors while still supporting users on legacy browsers.

Tech stack

JavaScriptNode.jsCloudflare

Getting it running

Difficulty · easy Time to first run · 5min

Hosted version requires just one script tag, self-hosting requires a separate repository and a Node.js server.

Open source under MIT, use freely, including in commercial projects, with attribution.

In plain English

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.

Copy-paste prompts

Prompt 1
I want to add Polyfill.io to my HTML site so older browsers get the features they need. Show me the script tag to add and explain what it does.
Prompt 2
I need to self-host the polyfill-service instead of using polyfill.io. Walk me through setting it up on a Node.js server.
Prompt 3
My site uses Polyfill.io but I only need polyfills for fetch and Promise. Show me how to request only those specific features in the service URL.
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