pr1.js

// Given a string "aaabbbccdeff", compress it, = "a3b3c2def2"

const input = "aaabbbccdeff";

function compressWord(str = '') {
  const size = (str || '').length;
  // edge cases
  if (size < 2) {
    return str;
  } else if (size === 2) {
    if (str.charAt(0) === str.charAt(1)) {
      return str.charAt(0) + '2';
    } else {
      return str;
    }
  }
  
  // count char occurrences
  let i = 1;
  const ch = str.charAt(0);
    
  while (i < str.length && ch === str.charAt(i)) {
    i++;
  }
  
  return str.substring(0, 1) 
    + (i > 1 ? i : '') 
      + compressWord(str.substring(i));
}

console.log(compressWord(input));