What is polyfilling and how to use it in JavaScript

Short answer

A polyfill is a script that implements a modern JavaScript feature in older environments that do not natively support it.

Steps

1. Feature-detect before polyfilling: javascript if (!Array.prototype.flat) { Array.prototype.flat = function(depth = 1) { /* implementation */ }; }
2. Use a service like polyfill.io for conditional delivery.
3. Use core-js with Babel for automatic polyfill injection: javascript import "core-js/stable";

Tips

• Only polyfill features you actually use.
• Prefer transpilation over polyfills when possible.

Common issues

• Over-polyfilling increases bundle size.
• Polyfills that do not fully match the spec cause subtle interoperability bugs.