In JavaScript, a polyfill is a piece of code that provides modern functionality to older browsers that lack native support for those features). 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). Polyfills can be used to mimic modern functionality on older browsers that do not natively support it, such as the Array.prototype.includes() method. However, polyfills can increase the size of a JavaScript bundle, which can impact performance.