freeCodeCamp Challenge Guide: Top rank per group

Guide
1

Top rank per group

Solutions

Solution 1 (Click to Show/Hide) 
const collectDept = function (arrOfObj, groupName) {
  const collect = arrOfObj.reduce((rtnObj, obj) => {
    if (rtnObj[obj[groupName]] === undefined) {
      rtnObj[obj[groupName]] = [];
    }
    rtnObj[obj[groupName]].push(obj);
    return rtnObj;
  }, {} // initial value to reduce
  );

  return Object.keys(collect).sort().map(key => collect[key]);
};

const sortRank = function (arrOfRankArrs, rankName) {
  return arrOfRankArrs.map(item => item.sort((a, b) => {
    if (a[rankName] > b[rankName]) { return -1; }
    if (a[rankName] < b[rankName]) { return 1; }
    return 0;
  }));
};

function topRankPerGroup(n, data, groupName, rankName) {
  if (n < 0) { return; }
  return sortRank(collectDept(data, groupName),
    rankName).map(list => list.slice(0, n));
}