JavaScript Run-length encoding

const getLengthOfOptiomalCompression = function(str) {
  if (str.length === 0) {
    console.log('Please enter valid string.');
    return;
  }

  let output = '';
  let count = '';
  for (let i = 0; i < str.length; i++) {
    count++;
    if (str[i] !== str[i+1]) {
      output += str[i] + count;
      count = 0;
    }
  }
  console.log(output);
}

getLengthOfOptiomalCompression('aaaa');
getLengthOfOptiomalCompression('aaaabbccddd');