groupBy(arr, func) on typescript

groupBy.ts

type FunctionTypeAsKey = number | string;
type ClusterFuncSignature<T> = (e: T) => FunctionTypeAsKey;
type Group<T> = {
    [k in FunctionTypeAsKey] : Array<T>
}

function groupBy<T> (arr : Array<T>, func : ClusterFuncSignature<T>) {

    const result = Object.create(null) as Group<T>;
    arr.forEach(elem => {
        let key : FunctionTypeAsKey = func(elem);
        const val : T = elem;
        key = (key === "__proto__" ? "_proto_" : key);
        (result[key] = result[key] || []).push(val);
    })

    Object.keys(result).forEach(key => result[key] === undefined ? delete result[key] : {}); // del all undefined props
    return {...result}
}

{
    const calcResult = groupBy([1.2, 1.1, 2.3, 0.4], Math.floor);
    const result = {
    "0": [0.4],
    "1": [1.2, 1.1],
    "2": [2.3],
    };
    console.log("eq first test: ", JSON.stringify(calcResult) === JSON.stringify(result));
}
{
    const calcResult = groupBy(["one", "two", "three"], (el) => el.length);
    let result = {
    "3": ["one", "two"],
    "5": ["three"],
    };
    console.log("eq second test: ", JSON.stringify(calcResult) === JSON.stringify(result));
}
{
    enum Gender {
    Male,
    Female,
    }
    const calcResult = groupBy(
    [
        { g: Gender.Male, n: "A" },
        { g: Gender.Female, n: "B" },
        { g: Gender.Female, n: "C" },
    ],
    (el) => el.g
    );
    let result = {
    [Gender.Male]: [{ g: Gender.Male, n: "A" }],
    [Gender.Female]: [
        { g: Gender.Female, n: "B" },
        { g: Gender.Female, n: "C" },
    ],
    };
    console.log("eq third test: ", JSON.stringify(calcResult) === JSON.stringify(result));
}
{
    const calcResult = groupBy([1,2,3], () => 'toString');
    let result = {
        "toString": [1,2,3]
    }
    console.log("eq fourth test: ", JSON.stringify(calcResult) === JSON.stringify(result));
}
{
    const calcResult = groupBy([1,2,3], () => '__proto__');
    let result = {
        "_proto_": [1,2,3]
    }
    console.log("eq fifth test: ", JSON.stringify(calcResult) === JSON.stringify(result));
}