1. querySelector

✍ example

HTML

<div id="login-form">
  <input type="text" placeholder="What is your name?" />
  <button type="button">로그인</button>
</div>

JS

const loginForm = document.getElementById("login-form");
const loginInput = loginForm.querySelector("input");
const loginButton = loginForm.querySelector("button");

또는

const loginInput = document.querySelector("#login-form input");
const loginButton = document.querySelector("#login-form button");