Vanilla ES6 functions to group database result sets, and example usage.

grouping.js

returnedUsers = {
  "users": [
    {
      "name": "dusan",
      "id": 1,
      "skill": "node.js",
      "skill_id": 1
    },
    {
      "name": "dusan",
      "id": 1,
      "skill": "php",
      "skill_id": 2
    },
    {
      "name": "dusan",
      "id": 1,
      "skill": "mongodb",
      "skill_id": 3
    },
    {
      "name": "jaca",
      "id": 2,
      "skill": "angular",
      "skill_id": 4
    },
    {
      "name": "jaca",
      "id": 2,
      "skill": "reactjs",
      "skill_id": 5
    },
  ]
};

const groupBy = (array, key) =>
  array.reduce((a, c) => (
    {
      ...a,
      [c[key]]: [...a[c[key]] || [], c]
    }
  ), {});

const uniques = (...values) =>
  Array.from(new Set([].concat(...values).filter(Boolean)));

const singularize = array =>
  array.length == 1 ? array[0] : array;

const singularizedUniques = (...values) =>
  singularize(uniques(...values));

const mergeCollect = array =>
  array.reduce((mergedObject, curentObject) => 
    Object.entries(curentObject).reduce((newObject, [k, v]) => (
      {
        ...newObject,
        [k]: singularizedUniques(newObject[k], v)
      }
    ), mergedObject)
  , {});

 const groupByKey = (array, key) =>
   Object.fromEntries(Object.entries(groupBy(array, key)).map(([k, v]) => [k, mergeCollect(v)]));

groupByKey(returnedUsers.users, 'id');