style.js

let usersData

function studentsByGroup(students) {
  fetch("https://mocki.io/v1/fcf2152e-707f-43c4-bb50-43c2ff29e697")
    .then((response) => response.json())
    .then((data) => (usersData = data));
}

studentsByGroup();

setTimeout(() => {
  console.log(usersData)
  const over18Users = usersData.filter(user => user?.age >= 18)
  const result = over18Users.reduce((userData, currentUser) => (
    { ...userData, [currentUser.group]: [currentUser] }), {}
  )
  console.log(result)
}, "2000");


/* expected output: arrToObj(payload) ->

{
  "1a": [
    {
      "group": "1a",
      "age": 22,
      "name": "John"
    }
  ],
  "2b": [
    {
      "group": "2b",
      "age": 18,
      "name": "Alise"
    }
  ],
  "2c": [
    {
      "group": "2c",
      "age": 19,
      "name": "Alex"
    }
  ]
}


*/