custom-function/esm/index.js

Literally the only sane way, if not the fastest one, to extend the Function class without evaluation

const {setPrototypeOf} = Object;

/**
 * Given a `class Test extends CustomFunction {}` a `new Test(() => {})`
 * will returns the provided callback as `instanceof Test`.
 * @param {function} fn the callback that will inherit class methods.
 * @returns {function} the same, yet upgraded, `fn` callback.
 */
export default function CustomFunction(fn) {
  return setPrototypeOf(fn, new.target.prototype);
}

CustomFunction.prototype = Function.prototype;