Proxy.es6.js

let target = {
  foo: "Welcome, foo"
}
let proxy = new Proxy(target, {
  get (receiver, name) {
      // get value from target if proprty exists else some logic from proxy
      return name in receiver ? receiver[name] : `Hello, ${name}`
  },
  // set value to target by adding new property
  set (receiver, name, value) {
    console.log(receiver, name, value);
    receiver[name]  = value;  
  }
})
proxy.foo   === "Welcome, foo"
proxy.world === "Hello, world"
proxy.tt = 10

console.log(target)