const people = [
  { name: "Kyle", age: 26 },
  { name: "John", age: 31 },
  { name: "Sally", age: 42 },
  { name: "Jill", age: 42 },
];
// reduce people array down to an individual result which groups by age
const result = people.reduce((groupedPeople, person) => {
    const age = person.age;
    // if theres no one with this age, create array
    if (groupedPeople[age] == null) groupedPeople[age] = [];
    // add person to array by age
    groupedPeople[age].push(person);
    // return all people grouped 
    return groupedPeople;
}, {})

console.log(result)