hash_map.js

const items = [{ key: 'a', value: 1 }, { key: 'a', value: 2 }, { key: 'b', value: 3 }, { key: 'b', value: 4 }];

const hashMap = items.reduce((acc, current) => {
    if (!acc[current.key]) acc[current.key] = [];
    acc[current.key].push(current.value);
    return acc;
}, {});

console.log(hashMap); // { a: [ 1, 2 ], b: [ 3, 4 ] }