<form id="idform">

        <input type="id" name="id" id="id_" placeholder="아이디를 입력해주세요...">

    </form>

    <h2>패스워드</h2>

    <form id="passform">

        <input type="password" name="pass" id="pass_" placeholder="비밀번호를 입력해주세요...">

    </form>

    <br>

    <br>

    <button id="submit">로그인</button>

    <!--JAVASCRIPT START-->

    <script> //JS

        document.getElementById('submit').onclick = function() {

            var ID = document.getElementById('id_').value;

            var pass = document.getElementById('pass_').value;

            if (ID === "1234" && pass === "1234") {

              alert('로그인 성공 메인화면으로 이동합니다!');

          window.location = 'index.html';

            }

            else {

                alert("로그인 실패!");

            }

        }

    </script>