What is "polyfill"

n. (context computing English) A plugin that provides the functionality of newer browsers to older versions.

In web development, a polyfill is code that implements a feature on web browsers that do not support the feature. Most often, it refers to a JavaScript library that implements an HTML5 web standard, either an established standard (supported by some browsers) on older browsers, or a proposed standard (not supported by any browsers) on existing browsers. Formally, "a polyfill is a shim for a browser API".

Polyfills allow web developers to use an API regardless of whether it is supported by a browser or not, and usually with minimal overhead. Typically they first check if a browser supports an API, and use it if available, otherwise using their own implementation. Polyfills themselves use other, more supported features, and thus different polyfills may be needed for different browsers. The term is also used as a verb: polyfilling is providing a polyfill for a feature.