如何提交判断text是空就弹出一个窗口警告？

1<script>   
2function checkform(){   
3if (form1.txt1.value==""){   
4alert("不能为空");   
5return false;   
6}   
7}   
8</script>

1<form id="form1" onsubmit="return checkform();">
2<input name="txt1" type="text"/>
3<input name="ok" type="submit" value="submit"/>
4</form>

1<script language="vbscript">   
2function checkform()   
3if (form1.txt1.value=="") then   
4msg("不能为空")   
5end if   
6</script>

 1<script>   
 2function checkform()   
 3dim errflag, msg   
 4errflag = True   
 5  
 6If len(trim(theForm.text1.value))= 0 then   
 7msg = "不能为空"   
 8MsgBox msg, 64, "警告！"   
 9focusto(0)   
10errflag = false   
11Exit Function   
12End if   
13  
14checkform = errflag   
15theForm.Submit   
16End Function   
17  
18sub focusto(x)   
19document.theForm.elements(x).focus()   
20end sub   
21</script>

1<form name="theform">
2<input name="txt1" type="text"/>
3<input name="ok" type="button" value="submit"/>
4</form>