提示框

<script type="text/javascript">
  //提示框prompt()第二个参数是默认值
 var x = prompt('enter number between 0 and 10','');
 try{
  if(x <0){
  //throw 把这个错误跑出去
   throw 'err1';
  }else if(x>10){
   throw 'err2';
  }
 }catch(err){
  if(err == 'err1'){
   alert('the value is too low');
  }else if(err == 'err2'){
   alert('the value is too high');
  }
 }
  </script>