Answer

A polyfill is a piece of JavaScript code that implements functionality that is missing in an environment. It allows developers to use certain APIs in environments that do not natively support them.

For example, if an environment does not support Array.prototype.myMap, we can implement a custom version that behaves similarly to the native map() method.

if (!Array.prototype.myMap) {
  Array.prototype.myMap = function (callback, thisArg) {
    const result = [];

    for (let i = 0; i < this.length; i++) {
      if (i in this) {
        result.push(callback.call(thisArg, this[i], i, this));
      }
    }

    return result;
  };
}

console.log([1, 2, 3].myMap(n => n * 2));
// [2, 4, 6]

Important points

• A polyfill implements missing functionality using JavaScript.
• A polyfill generally aims to reproduce the behavior of a native API, including its edge cases.
• A polyfill is different from a transpiler. A transpiler transforms source code, whereas a polyfill provides missing runtime functionality.
• Production polyfills should follow the relevant ECMAScript specification as closely as practical.