function groupPeople(people) {
    
    if (people !== null && Array.isArray(people) && Array(people).length !== 0 && typeof people[0] === 'object') {

        let groupedPeopleByAge = people.reduce((group, person) => {
            const { age } = person; //destructuring
            group[age] = group[age] ?? []; //nullish coalescing operatorn
            group[age].push(person.name);
            return group;
        }, {});

        return groupedPeopleByAge;
    }
    return [];
}

let people = [
    { name: "Alice", age: 25 },
    { name: "Bob", age: 25 },
    { name: "Charlie", age: 30 },
    { name: "David", age: 30 },
    { name: "Eve", age: 25 }
];

let groupedPeopleByAge = groupPeople(people);
console.log(groupedPeopleByAge);