(pretty) generic aggregation / grouping function using reduce

aggregation.ts

function aggregate<I, V>(
    items: I[],
    valueFun: (i: I) => V,
    groupingFun: (i: I) => string,
    combineFun: (v1: V, v2: V) => V
): ({ [g:string]: V }) {
     
    return items.reduce(
        (r: { [g: string]: V }, i: I) => {
            let g = groupingFun(i);
            let v = valueFun(i);
            r[g] = (r[g] != null) ? combineFun(v, r[g]) : v; 
            return r;
        },
        {}
    );
}

// sum of even / odd numbers
console.log(aggregate(
    [1, 2, 3, 4, 5, 6, 7, 8, 9],
    v => v, v => '' + (v % 2), (u, v) => (u + v)
));

// sum of count by category
console.log(aggregate(
    [
    { cat: 'a', value: 1 },
    { cat: 'a', value: 2 },
    { cat: 'b', value: 1 },
    { cat: 'b', value: 2 },
    { cat: 'b', value: 3 },
    { cat: 'c', value: 1 },
    { cat: 'b', value: 4 },
    { cat: 'a', value: 3 },
    ],
    v => v.value, v => v.cat, (u, v) => (u + v)
));