ob.js

function NamedOne(first, last) {
    this.firstName = first;
    this.lastName = last;
    Object.defineProperty(this, 'fullName',{
      get: function () {return this.firstName + ' ' + this.lastName;},
      set: function (value) {
        var splittedString = value.split(' ');
 
        if(splittedString.length === 2) {
          this.firstName = splittedString[0];
          this.lastName = splittedString[1];
        }
      }
    });
}
 
x = new NamedOne('John', 'Cat');
x.firstName; //John
x.fullName; //Juan Cat
 
x.firstName = 'Micaela'; //Micaela
x.fullName; //Micaela Cat
 
x.fullName = 'Amanda Dog';
x.firstName; // Amanda
x.lastName; // Dog