[рони]

Tanya51,

var users=[
  {
    firstname: 'Tanya',
    lastname: 'Sashyna',
    age: 25
  },
   {
    firstname: 'Lena',
    lastname: 'Velichko',
    age: 27
  },
  {
    firstname: 'Nastya',
    lastname: 'Sash',
    age: 5
  }
]
users.forEach(function(item, i, users) {
  item.fullName = item.firstname+" "+item.lastname;
});

console.log(users)