[Белый шум]

querubin,

function str_pack(arr) {
  var acc = 0, str = '';
  for( i in arr ){
      if( isNaN(arr[i]) )
          str = str + (acc > 0 ? acc : '') + arr[i],
          acc = 0;
      else
          acc = +arr[i] + acc;
  }
  if( acc > 0 ) str += acc;
  return str;
}
console.log(str_pack([1,1,'Q',1,'R']));
console.log(str_pack([1,1,1,'Q',1,1,'R',1]));
console.log(str_pack([1,1,'N','W',1,1,1,'Q']));