Compression of a string

strCompress

function compressStr(str){
  var index = 0;
  var counter = 1;
  var starter = str.charAt(0);
  var newStr = '';
  
  while(index -1 < str.length){
    if(starter === str.charAt(index+1)){
      counter ++;
    } else{
      newStr += (starter + counter);
      counter = 1;
      starter = str.charAt(index+1);
    }
    index++;
  }
  if (newStr.length < str.length){
    return newStr;
  } else{
    return str;
  }
}


compressStr('aaabbbccd')