This is my implementation of countingsort.

countingSort.js

function countingSort(arr){
    const aux = Array.from({length: Math.max(...arr)+1}, (v)=> 0)
    for(let i = 0; i<arr.length; i++){
      aux[arr[i]] = ++aux[arr[i]];
    }
    for (let i = 1; i < aux.length; i++) {
      let prev = aux[i-1];
      let next = prev + aux[i];
      aux[i] = next;
    }

    const list = Array.from({length: aux[aux.length-1]})

    for(let i = 0; i < list.length; i++){
      const item = arr[i];

      list[aux[item]-1] = item;
      aux[item] = --aux[item];
    }

    return list;
}

countingSort([4,9,2,4,7,6,11]);
// (7) [ 2, 4, 4, 6, 7, 9, 11 ]