In software development, a polyfill is code that implements a new standard feature of a deployment environment within an old version of that environment that does not natively support the feature. Most often, it refers to JavaScript code that implements an HTML5 or CSS 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. Polyfills are also used in PHP and Python.[1]
Polyfills allow web developers to use an API regardless of whether or not it is supported by a browser, 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.[2][3] 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.