Map reduce aggregation.js

let groupBy = 'date';

let apiData = [{date: 3, count: 5},{date: 2, count: 3},{date: 1, count: 1},{date: 1, count: 2},{date: 2, count: 4},{date: 3, count: 6}];
let sortedApiData = apiData.sort((a, b) => (a[groupBy] > b[groupBy]) ? 1 : ((b[groupBy] > a[groupBy]) ? -1 : 0 ))
let dateArray = sortedApiData.map((el, idx) => el[groupBy]).filter((el, i, a) => el !== a[i-1]);

result = dateArray
  .map((dayNumber, idx) => 
    sortedApiData
      .filter((apiDataPoint, i) => 
        apiDataPoint[groupBy] === dayNumber
      )
  ) // array of arrays clustered by date
  .map((dateGroup, idx) =>
    dateGroup
      .reduce((accumulator, current) => ({
        date: accumulator.date,
        count: accumulator.count + current.count
      })
    )
  ) // array of objects reduced by count