<form action="https://www.baidu.com" onsubmit="return fc()">
    <input type="text" name="username" value="vvv" />
    <input type="password" name="password" />
    <button type="submit">登入</button>
</form>
<script>

    function fc() {
        var fs = true
        var inp1 = document.querySelectorAll("input")[0];
        var inp2 = document.querySelectorAll("input")[1];
        var myInp1 = inp1.value;
        var myInp2 = inp2.value;
        if (myInp1 == "zs") {
            if (myInp2 == "12345") {
                alert("輸入正確");
                return fs;
            }
            else {
                fs = false;
                alert("密碼錯誤");
                return fs;
            }
        }
        else {
            fs = false;
            alert("使用者名稱錯誤");
            return fs;
        }
    }

</script>