function groupBy groupBy<T, K>(list: T[],keyFn: (item: T) => K): Map<K, T[]> Groups items into a Map keyed by the value returned from keyFn. Examples Example 1 groupBy([1, 2, 3, 4], (n) => n % 2 === 0 ? "even" : "odd"); // Map { "odd" => [1, 3], "even" => [2, 4] } const people = [{ role: "admin", name: "A" }, { role: "user", name: "B" }]; groupBy(people, (p) => p.role); // Map { "admin" => [{...}], "user" => [{...}] } Type Parameters T K Parameters list: T[] keyFn: (item: T) => K Return Type Map<K, T[]>