<textarea   rows=6   cols=20   onkeypress="check(this);"></textarea>   
  <script>   
  function   check(obj){   
  var   o=obj.createTextRange().getClientRects();   
  alert(o);
  }   
  </script>

alert(document.getElementById("taxtaera的ID").value.split(/\r?\n/).length);

<textarea   rows=6   cols=20   onkeypress="check(this);"></textarea>   
  <script>   
  function   check(obj){   
  var   o=obj.createTextRange().getClientRects();
  if(o.length>=10){
   event.returnValue=false;
   }
  }   
  </script>