Refactoring legacy mixin-based class hierarchies

Question

I'm currently working on a huge javascript project which has a huge class hierarchy and heavily uses mixins to extend functionality of base classes. Here is an example of how mixin looks like, we're using compose library to create class-like objects:

// Base.js
var Base = compose({
  setX: function (x) {
    this.x = x;
  },

  setY: function (y) {
    this.y = y;
  },

  setPosition: function (x, y) {
    this.setX(x);
    this.setY(y);
  }
})

// SameXAndY.js - mixin
var SameXAndY = compose({
  // Executes after setX in Base.js
  setX: compose.after(function (x) {
    this.y = x;
  }),

  // Executes after setY in Base.js
  setY: compose.after(function (y) {
    this.x = y;
  }),

  // Overrides setPosition in Base.js
  setPosition: compose.around(function (base) {
    return function (x, y) {
      if (x !== y) {
        throw 'x !== y';
      }
      return base.call(this, x, y);
    }
  })
})

We have the following problems with this solution:

• Mixins heavily depend on each other - you can break something by changing mixins' order in base classes.
• There is no easy way to ensure that you can safely include some mixin in your class, you may need to implement additional methods / include additional mixins into it.
• Child classes have hundreds of methods because of the various mixins.
• It's almost impossible to rewrite mixin in Flow or Typescript.

I'm looking for better plugin-like alternatives which allow to gradually refactor all existing mixins with the following requirements:

• Ability to explicitly describe all dependencies (i.e. somehow describe that PluginA requires PluginB and PluginC).
• Plugin should not pollute target class with its methods.
• They should be able to somehow intercept Base class logic (like in SameXAndY).
• Plugins should be plain js classes.

I understand that there is no "easy" answer to my question, but I would really love to hear your thought on this topic. Design pattern names, relevant blog posts, or even links to the source code will be greatly appreciated.

Cheers, Vladimir.

Answer 1

I did refactor the OP's example, and it does what was ask for. Let me know if I got something wrong.

class BaseXYType {

  constructor(stateValue) { // injected state object.

    this.setX = function setX (x) {
      return (stateValue.x = x);
    };
    this.setY = function setY (y) {
      return (stateValue.y = y);
    };

    Object.defineProperty(this, "x", {
      get: function getX () {
        return stateValue.x;
      },
      enumerable: true
    });
    Object.defineProperty(this, "y", {
      get: function getY () {
        return stateValue.y;
      },
      enumerable: true
    });

    Object.defineProperty(this, 'valueOf', {
      value: function valueOf () {
        return Object.assign({}, stateValue);
      }
    });
    Object.defineProperty(this, 'toString', {
      value: function toString () {
        return JSON.stringify(stateValue);
      }
    });
  }

  setPosition(x, y) { // prototypal method.
    this.setX(x);
    this.setY(y);
    return this.valueOf();
  }
}


class SameXYType extends BaseXYType {

  // - Traits in JavaScript should be applicable types/objects that are
  //   just containers of trait and object composition rules which
  //   exclusively will be executed at a trait's apply time.

  constructor(stateValue) { // injected state object.
    super(stateValue);

    withSameInternalXAndYState.call(this, stateValue);
  }
}


var withSameInternalXAndYState = Trait.create(function (use, applicator) {

  // local functions in order to enable shared code, thus achieving less memory consumption.
  //
  function afterReturningStateChangeXHandler(returnValue, argsArray, payloadList) {
    var
      stateValue = payloadList[0];

    stateValue.y = argsArray[0];  // same y from x.
  }
  function afterReturningStateChangeYHandler(returnValue, argsArray, payloadList) {
    var
      stateValue = payloadList[0];

    stateValue.x = argsArray[0];  // same x from y.
  }
  function setPositionInterceptor(proceedSetPosition, interceptor, argsArray, payloadList) {
    var
      x = argsArray[0],
      y = argsArray[1];

    if (x !== y) {
      throw (new TypeError([x, "!==", y].join(" ")));
    }
    return proceedSetPosition.call(this, x, y);
  }

  applicator(function sameXAndYBehavior (stateValue) {

    // no additional trait specific behavior within the applicator

  }).requires([

      "setX",
      "setY",
      "setPosition"

  ]).afterReturning(

    "setX", afterReturningStateChangeXHandler

  ).afterReturning(

    "setY", afterReturningStateChangeYHandler

  ).around(

    "setPosition", setPositionInterceptor
  );
});


var
  base_1 = new BaseXYType({ x: 7, y: 11 }),
  base_2 = new BaseXYType({ x: 99, y: 1 }),

  same_1 = new SameXYType({ x: 13, y: 5 }),
  same_2 = new SameXYType({ x: 99, y: 1 });


console.log('("" + base_1) : ', ("" + base_1));
console.log('("" + base_2) : ', ("" + base_2));
console.log('("" + same_1) : ', ("" + same_1));
console.log('("" + same_2) : ', ("" + same_2));

console.log('base_1.valueOf() : ', base_1.valueOf());
console.log('base_2.valueOf() : ', base_2.valueOf());
console.log('same_1.valueOf() : ', same_1.valueOf());
console.log('same_2.valueOf() : ', same_2.valueOf());


console.log('base_1.x : ', base_1.x);
console.log('(base_1.x = "foo") : ', (base_1.x = "foo"));
console.log('base_1.x : ', base_1.x);

console.log('base_1.y : ', base_1.y);
console.log('(base_1.y = "bar") : ', (base_1.y = "bar"));
console.log('base_1.y : ', base_1.y);

console.log('same_2.x : ', same_2.x);
console.log('(same_2.x = "biz") : ', (same_2.x = "biz"));
console.log('same_2.x : ', same_2.x);

console.log('same_2.y : ', same_2.y);
console.log('(same_2.y = "baz") : ', (same_2.y = "baz"));
console.log('same_2.y : ', same_2.y);


console.log('base_1.setY("foo") : ', base_1.setY("foo"));
console.log('base_1.y : ', base_1.y);

console.log('base_2.setX("bar") : ', base_2.setX("bar"));
console.log('base_2.x : ', base_2.x);


console.log('base_1.setPosition("brown", "fox") : ', base_1.setPosition("brown", "fox"));
console.log('("" + base_1) : ', ("" + base_1));

console.log('base_2.setPosition("lazy", "dog") : ', base_2.setPosition("lazy", "dog"));
console.log('("" + base_2) : ', ("" + base_2));


console.log('same_1.setY(543) : ', same_1.setY(543));
console.log('same_1.x : ', same_1.x);
console.log('same_1.y : ', same_1.y);
console.log('same_1.valueOf() : ', same_1.valueOf());

console.log('same_2.setY(79) : ', same_2.setY(79));
console.log('same_2.x : ', same_2.x);
console.log('same_2.y : ', same_2.y);
console.log('same_2.valueOf() : ', same_2.valueOf());


console.log('same_1.setPosition(77, 77) : ', same_1.setPosition(77, 77));
console.log('("" + same_1) : ', ("" + same_1));

console.log('same_2.setPosition(42, 42") : ', same_2.setPosition(42, 42));
console.log('("" + same_2) : ', ("" + same_2));


console.log('same_1.setPosition("apple", "pear") : ', same_1.setPosition("apple", "pear"));
console.log('("" + same_1) : ', ("" + same_1));

console.log('same_1.setPosition("apple", "pear") : ', same_1.setPosition("prune", "prune"));
console.log('("" + same_1) : ', ("" + same_1));

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