<form action="https://www.baidu.com/" onsubmit="return fn()">
    <input type="text" name="userName" />
    <input type="password" name="passWord" />
    <button type="submit" >點我</button>
</form>
<script>  
    function fn() {
        var flag=true;
        var myUserName = document.querySelectorAll("input")[0];
        var myPassWord = document.querySelectorAll("input")[1];
        if (myUserName.value == "zs") {
            if (myPassWord.value == "0819") {
                alert("驗證成功");
                return flag;
            }
            else {
                flag = false;
                alert("密碼錯誤，請重新輸入");
                return flag;
            }
        }
        else {
            flag = false;
            alert("使用者名稱錯誤，請重新輸入！");
            return flag;
        }
    }
</script>