Reproduce Vue's data object proxy behaviour

vue_experiment1.js

// https://vuejs.org/v2/guide/instance.html
//
//
//function Vue(opts){
  //for(var key in opts.data){
    //this[key] = opts.data[key]
  //}
//}
function Vue(opts){
  this._data = opts.data;
  Object.keys(opts.data)
    .forEach(key => {
      props = {
        configurable: true,
        enumerable: true,
        get() { return this._data[key]; },
        set(val) { this._data[key] = val; }
      }
      Object.defineProperty(this, key, props);
    });
}

/////////


let dataObj = {
  greetMsg: 'greet',
  byeMsg: 'bye'
}

let opts = {
  el: '#app1',
  data: dataObj
}
let vue = new Vue(opts);

console.log(vue.greetMsg === dataObj.greetMsg);

vue.greetMsg = 'modifiedGreet'
console.log(vue.greetMsg)
console.log(dataObj.greetMsg)

dataObj.greetMsg = 'yetAgainModifiedGreet'
console.log(dataObj.greetMsg)
console.log(vue.greetMsg)