What is polyfill-library?

Polyfill Library is a Node.js module that builds custom polyfill bundles based on the specific browser making a request. A polyfill is a small piece of code that adds a missing feature to an older browser so that modern JavaScript still works there. Instead of shipping every possible polyfill to every visitor, this library reads the browser's user agent string and returns only the polyfills that particular browser actually needs, keeping the bundle as small as possible. The main function is getPolyfillString, which takes a user agent string, a list of desired features, and some options, then returns either a minified or full size JavaScript bundle containing just the needed polyfills. Other functions let you list every polyfill available in the library, look up metadata about a specific polyfill by name, or build the full options object with its defaults filled in. Options include whether to minify the code, whether to treat unknown browsers as needing all polyfills or none, which features to exclude, and whether to include an anonymous usage reporting script in the bundle. The README also covers a specific setup for AWS Lambda functions, since Lambda expects files in a particular directory structure. It gives an example script that copies the polyfill files into the correct folder after a Lambda deployment is built, using the fs-extra and make-dir packages, so the polyfills are available at runtime. Development happens publicly on GitHub, and the project has a contributing guide explaining how to propose fixes, run tests, and use BrowserStack to test polyfills across real browsers. It is written in JavaScript and is released under the MIT license.