ECMAScript 6: automatically binding extracted methods (http://www.2ality.com/2013/06/auto-binding.html)

gistfile1.txt

// http://www.2ality.com/2013/06/auto-binding.html

(function(global) {
  const Function_bind = Function.prototype.call.bind(Function.prototype.bind);

  function autoBind(obj) {
    const handler = {
      get(target, name, receiver) {
        const result = Reflect.get(target, name, receiver);
        if (typeof result != 'function') {
          return result;
        }
        return Function_bind(result, receiver);
      }
    };
    return new Proxy(obj, handler);
  }

  const boundPrototypes = new WeakMap();
  function getBoundPrototypeFromConstructor(constructor) {
    const proto = constructor.prototype;
    if (!boundPrototypes.has(proto)) {
      boundPrototypes.set(proto, autoBind(proto));
    }
    return boundPrototypes.get(proto);
  }

  class BoundClass { }
  BoundClass[getSym("@@create")] = function() {
    return Object.create(getBoundPrototypeFromConstructor(this));
  }
  Object.defineProperty(BoundClass, getSym("@@hasInstance"), {
    value(obj) {
      if (obj != null && typeof obj == 'object') {
        const boundProto = getBoundPrototypeFromConstructor(this);
        if (boundProto === Reflect.getPrototypeOf(obj)) {
          return true;
        }
      }
      return Function.prototype[getSym("@@hasInstance")].call(this, obj);
    }
  });

  class Person extends BoundClass {
    constructor(name) {
      this.name = name;
    }
    describe() {
      return `I'm ${this.name}`;
    }
  }

  class Employee extends Person {
    constructor(name, title) {
      super(name);
      this.title = title;
    }
    describe() {
      return `${super()} (${this.title})`;
    }
  }

  global.Person = Person;
  global.Employee = Employee;

  // tests

  let jill = new Person('Jill');
  let jillDescribe = jill.describe;
  let john = new Employee('John', 'CFO');
  let johnDescribe = john.describe;

  print(jill.describe()); // prints "I'm Jill"
  print(jillDescribe());  // prints "I'm Jill"

  print(john.describe()); // prints "I'm John (CFO)"
  print(johnDescribe());  // prints "I'm John (CFO)"

  print(jill instanceof Person && !(jill instanceof Employee)); // true
  print(john instanceof Person && john instanceof Employee); // true
})(this);