creating hard-bound methods on classes

1.js

class Foo {
   constructor(x) { this.foo = x; }
   hello() { console.log(this.foo); }
}

class Bar extends Foo {
  constructor(x) { super(x); this.bar = x * 100; }
  world() { console.log(this.bar); }
}

bindMethods(Foo.prototype);
bindMethods(Bar.prototype);

var x = new Foo(3);
x.hello();  // 3
setTimeout(x.hello,50);   // 3

var y = new Bar(4);
y.hello();  // 4
y.world();  // 400
setTimeout(y.hello,50);   // 4
setTimeout(y.world,50);   // 400

2.js

var method = (function defineMethod(){
   var instances = new WeakMap();

   return function method(obj,methodName,fn) {
      Object.defineProperty(obj,methodName,{
         get() {
            if (!instances.has(this)) {
               instances.set(this,{});
            }
            var methods = instances.get(this);
            if (!(methodName in methods)) {
               methods[methodName] = fn.bind(this);
            }
            return methods[methodName];
         }
      });
   }
})();

function bindMethods(obj) {
   for (let ownProp of Object.getOwnPropertyNames(obj)) {
      if (typeof obj[ownProp] == "function") {
         method(obj,ownProp,obj[ownProp]);
      }
   }
}