SOURCE

class Watcher {
  constructor(proxy, key, cb) {
    this.cb = cb;
    Dep.target = this;
    this.value = proxy[key];
  }
}

class Dep {
  constructor(){
    this.subs = []
  }
  addSub(sub){
    this.subs.push(sub);
  }
  notify(newVal){
    this.subs.forEach(sub => {
      sub.cb(newVal, sub.value);
      sub.value = newVal;
    })
  }
}

const observe = (obj) => {
  const deps = {};
  return new Proxy(obj, {
    get: function (target, key, receiver) {
      const dep = (deps[key] = deps[key] || new Dep);
      Dep.target && dep.addSub(Dep.target)
      Dep.target = null;
      return Reflect.get(target, key, receiver);
    },
    set: function (target, key, value, receiver) {
      const dep = (deps[key] = deps[key] || new Dep);
      Promise.resolve().then(() => {
        dep.notify(value);
      })
      return Reflect.set(target, key, value, receiver);
    }
  });
}

var state = observe({
    x: 0,
    modifyX() {
        this.x = 10;
    }
});
new Watcher(state, 'x', function(n, o){
  console.log(n, o)
});
new Watcher(state, 'y', function(n, o){
  console.log(n, o)
});
state.x = 3;
state.y = 5;
state.modifyX();