CONST Example under ES6

const_function

function CONST (obj, key, value){
  Object.defineProperty(obj, key, {
    enumerable: false,
    configurable: false,
    writable: false,
    value: value
  });
  
}

function Person (name, birthday) {
  this.name = name;
  CONST(this, 'birthday', birthday);
}

// Create Person
var Thuya = new Person("Thuya", "31,Jan");
// Reassign
Thuya.birthday = "3,Feb";
// Checking
console.log(Thuya.birthday === "3,Feb");