Mixins for ES6 classes, transpiled with babel

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

Answers:

Answer

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.

Answer

Subclass Factory Mixins

There is another way to realize mixins in Javascript: With subclass factories.

A subclass factory is a function that excepts a base class and returns an extended subclass of this base class:

const mixin = base => class extends base {
  /* properties to mix in */
}

Subclass factories are possible for two reasons:

  • classes can be defined as expressions and are first class* in Javascript
  • extends clauses can contain arbitrary expressions

Let's apply this pattern:

// superclass
class CartoonCharacter {
  constructor(author) { this.author = author }
  drawnBy() { return "drawn by " + this.author }
}

// mixin 1
const Human = base => class extends base {
  haveFun() { return "drinking beer" }
}

// mixin 2
const Simpson = base => class extends base {}

// composed subclass
const Homer = Simpson(Human(CartoonCharacter));

// create an instance
const homer = new Homer("Matt Groening")

console.log(homer.drawnBy());
console.log(homer.haveFun());

What are the advantages of mixins by sublcass factories?

  • natural property precedence: Later mixed in properties override prior properties of the same name
  • both mixins (Human/Simpson) and subclasses (Homer) can use super as usual
  • mixins can have constructors (stateful mixins)
  • no mutation of prototypes or instances

Stateful Mixins

State makes things hard. You should avoid state, whenever possible. However, sometimes your mixin needs its own state, which is passed through a constructor:

class CartoonCharacter {
  constructor(author) { this.author = author }
  drawnBy() { return "drawn by " + this.author }
}

const Human = base => class extends base {
  haveFun() { return "drinking beer" }
}

const Simpson = base => class extends base {
  constructor(arg, ...superArgs) {
    super(...superArgs);
    this.name = arg;
  }
  sayHey() { return "Hey, I am " + this.name }
}

const Homer = Simpson(Human(CartoonCharacter));
const homer = new Homer("Homer Simpson", "Matt Groening")

console.log(homer.drawnBy());
console.log(homer.haveFun());
console.log(homer.sayHey());

*the term first class functions means that functions can be passed around as arguments or return values like normal data

Answer

As it has been discussed before by @mfeineis, @aadit-m-shah and @gotofritz as well as shown with the latest post from @ftor, one always needs to think carefully of witch class going to cripple into a "mixin" and than how to (re)assemble them into a hierarchy that still could be considered a good design. Such examples just show, that JavaScript might be in need for something that comes closer to traits than what we already (forced to) use instead. Something like ...

Applicable Objects/Types

  • that do not intermingle theirs syntax with that of classes,
  • because they are supposed to work exclusively at object/instance level, thus also from within (class) constructors and from within the (function) bodies of other applicable types, ...
  • therefore just being containers of both Trait and Object Composition Rules which exclusively will be executed at such a trait's/ApplicableType's apply time.

In order to prove such a concept in practice I did refactor all of the above examples into composition of trait from trait and into object composition from traits inside class constructors.

I would like discussing such an approach, its advantages and its shortcomings in comparison with all of the already accepted techniques that are in use today.

// - stateful applicable object/type proxy that does
//   link `this` context and injected local state.
//
const withExposeInternalStateProxy = Trait.create(function (use, applicator) {

  applicator(function (stateValue) {

    this.valueOf = function valueOf () {
      return Object.assign({}, stateValue);
    };
    this.toString = function toString () {
      return JSON.stringify(stateValue);
    };
  });
});


// - applicable object/type that just implements plain behavior.
// - also known as: function based mixin ... kind of "FLight Mixin reloaded".
//
const withHumanBehavior = Trait.create(function (use, applicator) {

  function allowWeekness() {
    return "allow weekness";
  }
  function showStrength() {
    return "show strength";
  }
  function thinkFirst() {
    return "think first";
  }
  function drinkingBeer() {
    return "drinking beer";
  }
  applicator(function () {

    this.allowWeekness = allowWeekness;
    this.showStrength = showStrength;
    this.thinkFirst = thinkFirst;
    this.drinkingBeer = drinkingBeer;
  });
});

// - composite trait that uses its trait composition language ... trait from trait(s).
//
const withStrongBeerDrinkingHabit = Trait.create(function (use, applicator) {

  use(withHumanBehavior)
    .apply(withHumanBehavior, "drinkingBeer").as("haveFun");

  //.apply.all.without("thinkFirst");
  //.apply(withHumanBehavior).without("thinkFirst");
});

// - trait that defines a specialized use case of another behavior.
//
const withDoAcceptAlreadyExistingDrinkingHabit = Trait.create(function (use, applicator) {
  applicator(function () {
    var
      type = this;

    if (typeof type.haveFun !== "function") {
      withStrongBeerDrinkingHabit.call(type);
    }
  });
});


class CartoonCharacter {

  constructor(stateValue) {

    // linking `this` context and injected local state.
    withExposeInternalStateProxy.call(this, stateValue);
  }
  drawnBy() {
    return ("drawn by " + this.valueOf().author);
  }
}

class MaleSimpsonsType extends CartoonCharacter {

  constructor(stateValue) {
    super(stateValue);

    // object/type composition
    //
    withDoAcceptAlreadyExistingDrinkingHabit.call(super.constructor.prototype);

    // withStrongBeerDrinkingHabit.call(super.constructor.prototype); // do not always apply.
    //
    // withStrongBeerDrinkingHabit.call(this); // nope ...prototype is the far better choice.
  }
  sayHey() {
    return ("Hey, I am " + this.valueOf().name);
  }
}


const homer = new MaleSimpsonsType({ name: "Homer Simpson", author: "Matt Groening" });

console.log("\n");
console.log('(homer + "") : ', (homer + ""));
console.log('homer.valueOf() : ', homer.valueOf());

console.log("\n");
console.log('homer.drawnBy() : ', homer.drawnBy());
console.log('homer.haveFun() : ', homer.haveFun());
console.log('homer.sayHey() : ', homer.sayHey());

console.log("\n");
console.log('Object.keys(homer) : ', Object.keys(homer));
console.log('homer.constructor : ', homer.constructor);
console.log('Object.keys(homer.constructor.prototype) : ', Object.keys(homer.constructor.prototype));
console.log('homer.constructor.prototype : ', homer.constructor.prototype);
console.log('(homer instanceof MaleSimpsonsType) : ', (homer instanceof MaleSimpsonsType));
console.log('(homer instanceof CartoonCharacter) : ', (homer instanceof CartoonCharacter));
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For backing up the above example, there is another thread - Refactoring legacy mixin-based class hierarchies - where I also took the chance of refactoring the OP's given example. This one does feature after and around modification of plain object based behavior. The same approach/toolset that was used here, did meet there all of the OP's requirements. One now is able of working with classes, gets encapsulated state/data for free and also has stateful support for any kind of method modification.

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