const mates = [{
    name: 'Alex',
    age: 13
  },
  {
    name: 'Oleg',
    age: 14
  },
  {
    name: 'Anna',
    age: 12
  },
]

const matesWithFriends = mates.map(mate => {
  const friends = mates.filter(otherMate => otherMate.name !== mate.name).map(friend => friend.name)
  return {...mate, friends}
})

console.log(matesWithFriends)

    let updatedMates = mates.map(currentMate => (
        {
            ...mate,
            friends: mates
                .map(friend => friend.name)
                .filter(friend => friend !== currentMate.name)
        }
    ))