Compress a string such that 'AAABCCDDDD' becomes 'A3B1C2D4'

const compress = str => { //build a character frequency map const freqM = [...str].reduce((acc, val) => { acc[val] = acc[val] + 1 || 1; return acc; }, {}); //return str if length <= unique characters * 2 (ie, 'A' vs 'A1')  if (str.length <= Object.keys(freqM).length * 2) return str; //return the frequency map as a string otherwise  return Object.entries(freqM).flat().join(''); }