javascript class traits mixins composition

Mixin and Trait based Compostion in JS

I got some trouble with compostions and mixin. For examples, lets imagine that we have a AHero and Hero1 object. All heroes can move, so AHero.move() is a thing. And now, at a moment of the dev, I want to add the possibility to stun and beStunned. So I make a new object that look like this ( called stunTrait) :

{
  stunned : false,
  stun(),
  beStun,
}

And I decide to give to possibility to heroes to stun and beStunned. So I do a mixin :

class AHero extends stunTrait(superclass).

Now I want that a stunned hero can not move. So in Ahero :

move() {
  if (this.stunned) return;
  ...
}

Later I find out stun was a bad idea, so I decide to stop implementing the stunTrait. And now I must find all stunned things in my Ahero class, and this can be sometimes hard, and even harder if child class did some other things with this stun stuff.

Is there a way to avoid that ? A better design ?

class Object{
  constructor(x, y) {
    this.x = x;
    this.y = y;
  }
}

class AHero extends stunTrait(Object) {
  constructor(x, y, moveSpeed) {
    this.moveSpeed = moveSpeed;
  }

  move() {
    super.move();
    this.speed = moveSpeed;
  }
}

class hero1 extends AHero {
  constructor(x, y, moveSpeed) {}
}

let stunTrait = function(object) {
  return class extends object {
    constructor(...args) {
      super(...args);

      this.stunned = false;
    }

    move() {
      if (this.stunned) this.speed = 0;
    }
  }
}

let hero = new hero1(40, 50, 100);
hero.stunned = true;
hero.move();

// This work, bcs AHero.move() is called before stunTrait.move()
// but if for some reason, I decide that only hero1 should have stunTrait
// So : class hero1 extends stunTrait(AHero)
// stunTrait.move() will be called after, so stunned will not be applied
// How make compostion without breaking inheritance ?

Solution

The OP's given design decision targets the modification of the move behavior that already has been implemented from the class side.

Thus a stun behavior is not only responsible for implementing its stun specific state handling, it also has to decide whether and/or how to change a move behavior's control flow.

For this it relies on existing implementations not only of move but also on setSpeed.

The concept of mixins really does not fit with the OP's approach for it just mixes additional behavior into other objects. But the given special case has to handle the naming conflict of move too. And that's something what traits are for.

If, in JavaScript, there once was a real support for applicable types which have to be capable of both kinds of composition trait- and object-composition, as well as of method modification, a possible solution might look similar to the next provided one.

const withStunBehavior = Trait.create(function (use, applicator) {

  function moveInterceptor(proceedMove, interceptor, argsArray) {
  //console.log("withStunBehavior::moveInterceptor - this.isStunned() : ", this.isStunned());
    if (this.isStunned()) {

      this.setSpeed(0);
    }/* else {
      return proceedMove.call(this);
    }*/
    return proceedMove.call(this);
  }

  applicator(function (isStunnedValue) {
    isStunnedValue = !!isStunnedValue;
  //console.log("withStunBehavior::applicator - isStunnedValue : ", isStunnedValue);

    this.beStun = function beStun () {
      isStunnedValue = true;
    };
    this.unban = function unban () {
      isStunnedValue = false;
    };
    this.isStunned = function isStunned () {
      return isStunnedValue;
    };

  }).requires([

    "setSpeed",
    "move"

  ]).around(

    "move", moveInterceptor
  );
});


class AHero {
  constructor(stateValue) {
    var
      aHero = this;

    aHero.getSpeed = function getSpeed () {
      return stateValue.moveSpeed;
    };
    aHero.setSpeed = function setSpeed (speedValue) {
      return (stateValue.moveSpeed = speedValue);
    };

    // withStunBehavior.call(aHero);
  }
  move() {
    var
      speedValue = this.getSpeed();

    if (speedValue === 0) {
      console.log("Hero does not move at all. Its speed is " + speedValue + ".");
    } else {
      console.log("Hero moves with speed " + speedValue + ".");
    }
    return speedValue;
  }
}


class AHeroWithStunBehavior extends AHero {

  constructor(stateValue) {
    super(stateValue);

    withStunBehavior.call(this);
  }
}


var
  hero = new AHero({ x: 0, y: 0, moveSpeed: 200 });

console.log("\n");
console.log("hero.getSpeed() : ", hero.getSpeed());
console.log("hero.move() : ", hero.move());
console.log("\n");
console.log("hero.setSpeed(0) : ", hero.setSpeed(0));
console.log("hero.getSpeed() : ", hero.getSpeed());
console.log("hero.move() : ", hero.move());
console.log("\n");
console.log("hero.setSpeed(100) : ", hero.setSpeed(100));
console.log("hero.getSpeed() : ", hero.getSpeed());
console.log("hero.move() : ", hero.move());
console.log("\n");
console.log("hero.isStunned : ", hero.isStunned);
console.log("hero.beStun : ", hero.beStun);
console.log("hero.unban : ", hero.unban);

var
  stunningHero = new AHeroWithStunBehavior({ x: 40, y: 50, moveSpeed: 130 });

console.log("\n");
console.log("stunningHero.isStunned : ", stunningHero.isStunned);
console.log("stunningHero.beStun : ", stunningHero.beStun);
console.log("stunningHero.unban : ", stunningHero.unban);
console.log("\n");
console.log("stunningHero.getSpeed() : ", stunningHero.getSpeed());
console.log("stunningHero.move() : ", stunningHero.move());
console.log("\n");
console.log("stunningHero.setSpeed(0) : ", stunningHero.setSpeed(0));
console.log("stunningHero.getSpeed() : ", stunningHero.getSpeed());
console.log("stunningHero.move() : ", stunningHero.move());
console.log("\n");
console.log("stunningHero.setSpeed(90) : ", stunningHero.setSpeed(90));
console.log("stunningHero.getSpeed() : ", stunningHero.getSpeed());
console.log("stunningHero.move() : ", stunningHero.move());
console.log("\n");
console.log("stunningHero.beStun() : ", stunningHero.beStun());
console.log("stunningHero.move() : ", stunningHero.move());
console.log("\n");
console.log("stunningHero.setSpeed(160) : ", stunningHero.setSpeed(160));
console.log("stunningHero.getSpeed() : ", stunningHero.getSpeed());
console.log("stunningHero.move() : ", stunningHero.move());
console.log("\n");
console.log("stunningHero.unban() : ", stunningHero.unban());
console.log("stunningHero.move() : ", stunningHero.move());
console.log("\n");
console.log("stunningHero.setSpeed(160) : ", stunningHero.setSpeed(160));
console.log("stunningHero.getSpeed() : ", stunningHero.getSpeed());
console.log("stunningHero.move() : ", stunningHero.move());

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