输入框按回车,做了判断,可是还是跳转了~

Question

搜索页面我做了回车提交,在输入框为空的时候 弹出提示框,之后还是表单提交了 这是为什么呢?   <form name="search1" action="product_search.asp" method="post">
  <td><table>
    <tr>
      <td>
       <select name="id" class="myinput22">
         <option value="0" selected>Products Series:</option>
       </select>
      </td>
    </tr>
    <tr>
      <td><input name="keyword" type="text" onKeydown="keyDown();" onfocus="clearTip(this)" value="Products Name:"></td>
    </tr>
    <tr>
      <td><a style="cursor:pointer;" id="button" onclick="return search_onclick();"><img src="../images/s.jpg" width="82" height="28" border="0"></a></div></td>
    </tr>
  </table></td>
</form>
<SCRIPT language=javascript>             
<!--
function search_onclick()
{
if( document.search1.keyword.value=="Products Name:" )
{
  alert("Please enter the search content!");
return false;
}
else if( javaTrim(document.search1.keyword.value)=="" )
{
  alert("Please enter the search content!");
return false;
}
else{
//document.search1.submit();
return true;
}
}
function clearTip(field){
     if( field.value == field.defaultValue ) 
     {
     field.value = "";
     }
}
function keyDown() 
    { 
        var e=event.srcElement; 
       
        if(event.keyCode==13) 
        {
         document.getElementById("button").click();
        }
    }
//-->
</SCRIPT>

Answer 1

<form name="search1" action="product_search.asp" method="post">
  <td><table>
    <tr>
      <td>
          <select name="id" class="myinput22">
            <option value="0" selected>Products Series:</option>
          </select>
      </td>
    </tr>
    <tr>
      <td><input id="keyword" name="keyword" type="text" onKeydown="return keyDown();" onfocus="clearTip(this)" value="Products Name:"></td>
    </tr>
    <tr>
      <td><a style="cursor:pointer;" id="button" onclick="return search_onclick();"><img src="../images/s.jpg" width="82" height="28" border="0"></a></div></td>
    </tr>
  </table></td>
    </form>
    <SCRIPT language=javascript>             
    <!--
        function search_onclick() {
            if (document.search1.keyword.value == "Products Name:") {
                alert("Please enter the search content!");
                return false;
            }
            else if (javaTrim(document.search1.keyword.value) == "") {
                alert("Please enter the search content!");
                return false;
            }
            else {
                //document.search1.submit();
                return true;
            }
        }
        function clearTip(field) {
            if (field.value == field.defaultValue) {
                field.value = "";
            }
        }
        function keyDown() {
            var e = event.srcElement;            if (event.keyCode == 13) {
                if (document.getElementById("keyword").value != "") {
                    document.getElementById("button").click();
                    return true;
                }
                else {
                    return false;
                }
            }
        }
    //-->
    </SCRIPT>

Answer 2

keyDown（）无返回值
=------------------------
        function keyDown() 
    { 
        var e=event.srcElement; 
       
        if(event.keyCode==13) 
        {
          return document.getElementById("button").click();
        }
    }