// Вызов функции - groupBy(persons, "age")
 
const persons = [
  { name: "Alex", age: 20 },
  { name: "Lena", age: 25 },
  { name: "Pavel", age: 20 },
];
 
function groupBy(arr, prop) {
  // Ваш код здесь
}
 
console.log(groupBy(persons, "age"));
 
// Результат {
//   20: [{ name: 'Alex', age: 20 }, { name: 'Pavel', age: 20 } ],
//   25: [{ name: 'Lena', age: 25 }]
// }

function groupBy(arr, prop) {
  let groupRes = {};
 
  arr.forEach((el) => {
    const { name, age } = el;
 
    if (groupRes[age]) {
      groupRes[age].push({ name, age });
    } else {
      groupRes[age] = [{ name, age }];
    }
  });
  return groupRes;
}
 
console.log(groupBy(persons, "age"));