countSort.js

function countSort(array) {
  const counts = {};
  for(const num of array) {
    if(counts[num]) counts[num] += 1;
    else counts[num] = 1;
  }

  for(let idx = 0; idx < Object.keys(counts).length - 1; idx++) {
    counts[Object.keys(counts)[idx + 1]] += counts[Object.keys(counts)[idx]]; 
  } 

  const result = [];
  for(const num of array.reverse()) {
    result[counts[num] - 1] = num;
    counts[num] -= 1;
  }

  return result;
}

console.log(countSort([7,3,6,5,1])) // [1,3,5,6,7]