楼上，为什么startCount后面不用加（）

  var num=0;
  function startCount() {
    document.getElementById('count').value=num;
    num=num+1;
    setTimeout("startCount()",1000);
  }
  window.onload=startCount;

 
  <script type="text/javascript">
   function count(){
    var oTxt1 = document.getElementById("txt1").value;
    var oTxt2 = document.getElementById("txt2").value;
    var oSlt = document.getElementById("select").value;
    var result = "";
    switch(oSlt) {
        case "+":
            result = parseFloat(oTxt1) + parseFloat(oTxt2);
            break;
        case "-":
            result = parseFloat(oTxt1) - parseFloat(oTxt2);
            break;
        case "*":
            result = parseFloat(oTxt1) * parseFloat(oTxt2);
            break;
        default:
            result = parseFloat(oTxt1) / parseFloat(oTxt2);
    }
    document.getElementById("fruit").value = result;
    
   }
  </script>