callable-instances.js

// Experiment: I’m not sure if I would use this myself.

const call = Symbol('call');
class Callable {
  constructor() {
    // Can’t use .bind() here. Not sure why. Maybe the result doesn’t
    // interact well with Object.setPrototypeOf().
    const _this = (...args) => new.target.prototype[call].call(_this, ...args);
    Object.setPrototypeOf(_this, new.target.prototype);
    return _this;
  }
}

class Frobnicator extends Callable {
  constructor(x, y) {
    super();
    this.x = x;
    this.y = y;
  }
  [call](z) {
    return z * this._product;
  }
  // Can’t use private slots (which can only be accessed via original instance)
  get _product() {
    return this.x * this.y;
  }
}

const ten = new Frobnicator(2, 5);
console.log(ten(3), "thirty");

Looks like Callable can also be implemented as:

class Callable extends Function {
  constructor() {
    const _this = super('return this(...arguments)').bind((...args) => new.target.prototype[call].apply(_this, args));
    return _this;
  }
}

Any downsides?