Simple compression function

compressor.io

// Takes a sequence of letters and returns a compressed output
// input = abcaaabbb
// expected = abca3b3
const compressor = (str) => {
  if (typeof str !== "string" || !/^[A-Za-z]+$/.test(str)) {
    return "Input must be a string";
  }

  if (str.length > 5000) {
    return "Input must be less than 5000 chars.";
  }

  const arr = str.split("");
  let outputStr = "";

  for (let i = 0; i < arr.length; i++) {
    let sameLetter = false;
    if (i > 0) {
      sameLetter = arr[i] == arr[i - 1];
    }
    if (sameLetter) {
      let sameCount = 1;
      let iterCount = 1;
      while (arr[i] == arr[i + iterCount]) {
        sameCount++;
        iterCount++;
      }
      outputStr += sameCount + 1;
      i = i + iterCount - 1;
    } else {
      outputStr += arr[i];
    }
  }
  return outputStr;
};