function fnMethodName(x){ 
alert(x); 
}

<script type="text/javascript">

  function add(num1,num2)
    { 
    return num1+num2+200;
    }
  var sum=add(122,10000);
  window.document.write(sum);

</script> 

  <script type="text/javascript">

    var add=function(num1,num2)
      { return num1+num2+200;}
    var sum=add(122,300);
    window.document.write(sum);
  </script>

<script type="text/javascript">
    var add=new Function("num"," return num+200");//此时Function 这种方法用的比较少
    var sum=add(122);
    window.document.write(sum);

</script>