What does Object.groupBy return in this example?

const people = [
  { name: "Ada", age: 36 },
  { name: "Linus", age: 21 },
  { name: "Grace", age: 85 },
];
const groups = Object.groupBy(people, (person) => (person.age < 30 ? "young" : "senior"));
 
console.log(Object.keys(groups));
console.log(groups.senior.map((person) => person.name));
console.log(groups.adult);
console.log("toString" in groups);
console.log(groups.senior[0] === people[0]);