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groupBy: group elements

Write a function groupBy(array, getKey) that sorts elements into groups. The group key is getKey(item), and the value is an array of elements in their original order.

For example, groupBy([1, 2, 3, 4], (n) => (n % 2 ? "odd" : "even")) returns { odd: [1, 3], even: [2, 4] }.

function groupBy(array, getKey) {
  // ...
}

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Tests (4)

expect(groupBy([1, 2, 3, 4], (n) => (n % 2 ? "odd" : "even"))).toEqual({ odd: [1, 3], even: [2, 4] });

expect(groupBy(["one", "two", "three"], (s) => s.length)).toEqual({ 3: ["one", "two"], 5: ["three"] });

expect(groupBy([], (x) => x)).toEqual({});

const people = [{ name: "Ann", city: "Kyiv" }, { name: "Bob", city: "Warsaw" }, { name: "Eve", city: "Kyiv" }];
expect(groupBy(people, (p) => p.city).Kyiv.map((p) => p.name)).toEqual(["Ann", "Eve"]);