countingsort.js

const countsort = (array, k) => {
  const frequency = [...Array(k + 1).keys()].map(k => 0);
  const out = [];

  for(let j in array){
    frequency[array[j]] = frequency[array[j]] + 1;
  }
  
  for(let i = 1; i < frequency.length; i++){
    frequency[i] = frequency[i] + frequency[i - 1];
  }

  for(let i = array.length - 1; i >= 0; i--){
    out[frequency[array[i]] - 1] = array[i];
    frequency[array[i]] = frequency[array[i]] - 1;
  }

  return out;
};