<form name="form1" action="">
    <input type="text" name="username">
    <input type="button" name="submitForm" value="提交" onclick="submitForm1();">
</form>
<script>
    function submitForm1(){
        var result=window.showModalDialog("myconfirm.html",
            2,"dialogWidth:240px;dialogHeight:120px;status:no;");
        if(result==1){
            check(document.getElementsByName("form1")[0]);
        }
    }
    function check(frm){
        var username=document.getElementsByName("username")[0];
        var result;

        if(username.value==""){
            result=window.showModalDialog("myconfirm.html",
            new Array(1,"请输入用户名"),"dialogWidth:240px;dialogHeight:120px;status:no;");
            return;
        }
        frm.submit();
    }
</script>