Mixins for ES6 classes, transpiled with babel

Question

According to various sources (2ality, esdiscuss) one should be able to add mixins to classes:

EDIT discovered that class methods are not enumerable so that cannot work. Edited the code below, but still no joy

class CartoonCharacter {
  constructor(author) {
    this.author = author;
  }

  drawnBy() {
    console.log("drawn by", this.author);
  }
}

// THIS CANNOT WORK
// class methods are not enumerable
// class Human {
//  haveFun() {
//    console.log("drinking beer");
//  }
// }

let Human = Object.create({}, {
  haveFun:   {
    enumerable: true,
    value: function () {
      console.log("drinking beer");
    }
  }
});

class Simpson extends Object.assign(CartoonCharacter, Human) {
  constructor(author) {
    super(author);
  }
}


let homer = new Simpson("Matt Groening");
homer.drawnBy();  // expected: drawn by Matt Groening
homer.haveFun();  // expected: drinking beer

I get the "drawn by Matt Groening" but instead of the "drinking beer" I get an error

-> Uncaught TypeError: E.haveFun is not a function

---

Answer 1

There are two problems with your mixins:

1. Object.assign only copies enumerable properties of an object. However, the methods and properties of a class are non-enumerable.
2. The methods and properties of a class are not defined on the constructor. They are defined on the prototype of the constructor.

This is how you would extend a class using mixins:

class CartoonCharacter {
  constructor(author) {
    this.author = author;
  }

  drawnBy() {
    console.log("drawn by", this.author);
  }
}

class Human {
  haveFun() {
    console.log("drinking beer");
  }
}

mixin(CartoonCharacter, Human);

class Simpson extends CartoonCharacter {
  constructor(author) {
    super(author);
  }
}


let homer = new Simpson("Matt Groening");
homer.drawnBy();  // expected: drawn by Matt Groening
homer.haveFun();  // expected: drinking beer

function mixin(target, source) {
  target = target.prototype; source = source.prototype;

  Object.getOwnPropertyNames(source).forEach(function (name) {
    if (name !== "constructor") Object.defineProperty(target, name,
      Object.getOwnPropertyDescriptor(source, name));
  });
}

It works as expected in Babel: demo.