sp-polyfills
A "polyfill" is a script that emulates a modern JavaScript feature when it is not available in an older web browser. The sp-polyfills package provides standard polyfills for the specific feature set that the SharePoint Framework expects from its environment. It is acceptable for a browser environment (e.g. a unit test runner) to load its own polyfills, but they must be equivalent to the definitions in this package.
This package is part of the SharePoint Framework, which is a collection of NPM packages that empower developers to create client-side experiences for Microsoft SharePoint. For more information, including complete API documentation and code samples, please visit the SharePoint Framework web site.