en.javascript.info/1-js/6-objects-more/4-descriptors-getters-setters/1-replace-property-getter/solution.md

//+ run

function User(fullName) {
  this.fullName = fullName;

  Object.defineProperties(this, {

    firstName: {

      get: function() {
        return this.fullName.split(' ')[0];
      },

      set: function(newFirstName) {
        this.fullName = newFirstName + ' ' + this.lastName;
      }

    },

    lastName: {

      get: function() {
        return this.fullName.split(' ')[1];
      },

      set: function(newLastName) {
        this.fullName = this.firstName + ' ' + newLastName;
      }

    }


  });
}

var vasya = new User("Василий Попкин");

// чтение firstName/lastName
alert(vasya.firstName); // Василий
alert(vasya.lastName); // Попкин

// запись в lastName
vasya.lastName = 'Сидоров';

alert(vasya.fullName); // Василий Сидоров