groupBy.ts

const groupBy = <T extends { [k in K]: string }, K extends string>(
  arr: T[],
  attr: K,
) => {
  return arr.reduce<{ [k in T[K]]?: T[] }>((result, item) => {
    const k = item[attr];
    if (result[k] === undefined) {
      result[k] = [];
    }
    (result[k] as T[]).push(item);
    return result;
  }, {});
};

test("groupBy", () => {
  const data = [
    { type: "foo", id: 1 },
    { type: "foo", id: 2 },
    { type: "bar", id: 3 },
  ];
  expect(groupBy(data, "type")).toEqual({
    foo: [data[0], data[1]],
    bar: [data[2]],
  });
});