js中parseInt()数字相加

<input   type="text"   id="a1"   value="1">   +    
  <input   type="text"   id="a2"   value="1">   =    
  <input   type="text"   id="a3"   readonly="true">  
  <br>  
  <input   type="button"   value="click"   onclick="foo()">  
  <script>  
  function   foo(){  
  a3.value   =   parseInt(a1.value)   +   parseInt(a2.value);  
  }  
  </script>