Accessors Composition in ES6 Classes

Question

Let's say I have a Thing class which I want to be both Hideable and Openable.

Using a similar approach to Douglas Crockford's object creation through composition, I have been able to "inherit" from multiple classes.

This approach does not work with accessors (getter/setters).

I need to use classes as it's a requirement. I'm also finding that I am duplicating functionality from class to class, but I don't want these to inherit from a base class.

Any ideas?

The progress I have made so far is in the below snippet:

class Openable {

  constructor(isOpen = false) {
    this._isOpen = isOpen;
  }

  get isOpen() {
    return this._isOpen + ' is stupid.';
  }

  set isOpen(value) {
    this._isOpen = value;
  }

}


class Hideable {

  constructor(isHidden = false) {
    this._isHidden = isHidden;
  }

  get isHidden() {
    return this._isHidden + ' is stupid.';
  }

  set isHidden(value) {
    this._isHidden = value;
  }

}


class Thing {

  constructor(config) {
    let { isOpen, isHidden } = config;

    let openable = new Openable(isOpen);
    this.isOpen = openable.isOpen;

    let hideable = new Hideable(isHidden);
    this.isHidden = openable.isHidden;
  }

}


let thing = new Thing({
  isOpen: true,
  isHidden: false
});

Answer 1

Because isOpen and isHidden are accessors, you can't just grab a copy of them, you have to access them when you want them.

Still, you can create your own isOpen, isHidden which use the underlying ones:

let openable = new Openable(isOpen);
Object.defineProperty(this, "isOpen", {
    get: () => openable.isOpen,
    set: value => {
        openable.isOpen = value;
    }
});

let hideable = new Hideable(isHidden);
Object.defineProperty(this, "isHidden", {
    get: () => hideable.isHidden,
    set: value => {
        hideable.isHidden = value;
    }
});

Live example on Babel's REPL

Naturally, if you do this a lot, you'd want to have a worker function to set that up rather than retyping it all the time:

function wrapProperty(dest, src, name) {
    Object.defineProperty(dest, name, {
        get: () => src[name],
        set: value => { src[name] = value; }
    });
}

(or do it by grabbing the property descriptor and updating it)

then:

wrapProperty(this, openable, "isOpen");
wrapProperty(this, hideable, "isHidden");

---

I'd question the requirement that you must use class for Openable and Hideable. They look much more like mixins to me.