let vasya = { name: "", surname: "", id: 1 };
let petya = { name: "", surname: "", id: 2 };
let masha = { name: "", surname: "", id: 3 };

let users = [ vasya, petya, masha ];

let usersMapped = users.map(user => ({
  fullName: `${user.name} ${user.surname}`,
  id: user.id
}));

/*
usersMapped = [
  { fullName: " ", id: 1 },
  { fullName: " ", id: 2 },
  { fullName: " ", id: 3 }
]
*/

alert( usersMapped[0].id ); // 1
alert( usersMapped[0].fullName ); //  

let usersMapped = users.map(user => {
  fullName: `${user.name} ${user.surname}`,
  id: user.id
});

let usersMapped = users.map(user => ({
  fullName: `${user.name} ${user.surname}`,
  id: user.id
}));