zloirock/core-js

Core-js is a modular standard library for JavaScript that solves a common compatibility headache: different web browsers and JavaScript environments support different modern language features. When you write code using the latest JavaScript syntax and built-in tools, older browsers may not understand it. Core-js bridges that gap by providing "polyfills", replacement implementations of modern features that work in environments where those features don't exist natively. The library covers a huge range of JavaScript functionality, from Promises (a way to handle tasks that take time, like loading data) and Sets (collections of unique values) to newer additions in ECMAScript 2025, the annual standard that defines what JavaScript can do. You can load the entire library at once, or pick only the specific features you need to keep your code lightweight. It also offers a "pure" mode that adds features without touching global objects, which prevents conflicts when multiple libraries are used together. Developers typically use core-js when building websites that need to run in older browsers, when working with tools like Babel (a compiler that translates modern JavaScript to an older version), or when using the swc build tool. It's a foundational piece of infrastructure used by millions of JavaScript projects worldwide. The tech stack is JavaScript, and it integrates with tools like Babel's preset-env and runtime packages.