^{Social Media Photo by Aaron Huber on Unsplash}

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

// const CustomFunction = require('custom-function');
import CustomFunction from 'custom-function';

class MyFunction extends CustomFunction {
  invoke(...args) {
    return this(...args);
  }
  toString() {
    return '[native code]';
  }
}

const cf = new MyFunction((a, b) => a + b);
cf(1, 2);         // 3
cf.invoke(1, 2);  // 3
cf.toString();    // "[native code]"

Object.assign(fn, {...extras}) can augment fn but it cannot have accessors, private fields, super.method(...args) calls, and so on, while with this module, everything possible with classes is possible with functions too.

Object.defineProperties can have accessors and not all enumerable and writable fields, but it still cannot set private fields or allow super.method(...args) calls.

On top of these limitations, adding O(n) features requires more effort than swapping once the prototypal chain, where the original Function.prototype root of the chain will be preserved regardless.

As summary: every solution to date that is not based on native class extends feature is somehow limited, likely slower, surely inferior in terms of DX.

While writing this I had Function as one problematic constructor that cannot be extended due inevitable code evaluation involved while invoking super() but it's true that this approach/pattern can be used for any constructor that could be problematic if invoked right away, which is why there is a custom-function/factory export too so that anything becomes possible, example:

// const custom = require('custom-function/factory');
import custom from 'custom-function/factory';

// reproduce exactly what this module provides:
const CustomFunction = custom(Function);
class MyFunction extends CustomFunction {}

// go wild with any illegal constructor too
const Div = custom(HTMLDivElement);
class MyDiv extends Div {
  constructor(...childNodes) {
    super(document.createElement('div'));
    this.append(...childNodes);
  }
}

document.body.appendChild(
  new MyDiv(
    new MyDiv('A'),
    new MyDiv('B', 'C')
  )
);

• custom-function to extend functions without invoking Function
• custom-function/factory to extend any class without invoking the extended class constructor
• custom-function/closure to extend functions in a Closure Compiler friendly way
• custom-function/closure-factory to extend any class in a Closure Compiler friendly way

The benchmark test properties added to a callback using this module VS using a predefined descriptors object via Object.defineProperties to compare the most possible perf-tuned define properties approach against this module pattern.

Use npm run bench to test yourself locally, after cloning this repo.

cold run
CustomFunction
  creation: 20.884ms
  metohd: 3.421ms
Object.defineProperties
  creation: 76.917ms
  metohd: 4.838ms

hot run
CustomFunction
  creation: 7.597ms
  metohd: 0.667ms
Object.defineProperties
  creation: 94.064ms
  metohd: 1.359ms