QUESTION: What is a polyfill?

Even after successful code transpilation, older browsers may face the issue of lacking support for certain global objects, methods, or functions introduced in newer language specifications. This is where a polyfill comes to the rescue—it is a special piece of code that adds missing functionality to an environment that does not natively support it. If an older browser does not know what promises or the includes array method are, the polyfill checks for the existence of this function and, if it is missing, implements it in a standard way. Thus, your code continues to work equally well in both the most modern mobile browser and an outdated desktop system.

Historically, developers used various libraries to include missing features, but today the industry standard is the core-js library, which contains a comprehensive set of polyfills for virtually any modern API. Clear examples of the need for such solutions are Promise objects for working with asynchronous code and the fetch function for making network requests in legacy environments. Previously, the @babel/polyfill package was actively used to automate this process, but today it is officially deprecated and no longer recommended for use. Instead, the modern ecosystem offers flexible configuration of core-js combined with the Babel compiler, allowing you to automatically include only those polyfills that are truly necessary for your project's target browsers, significantly saving on total download traffic.