HTMLFieldSetElement: свойство elements

<form id="my-form">
  <fieldset id="my-fieldset">
    <legend>My fieldset</legend>
    <p>
      <label for="username">Username:</label>
      <input type="text" id="username" name="username" />
    </p>
    <p>
      <label for="password">Password:</label>
      <input type="password" id="password" name="password" />
    </p>
    <p>
      <input type="checkbox" id="remember-me" name="remember-me" />
      <label for="remember-me">Remember me</label>
    </p>
  </fieldset>
</form>

const fieldset = document.getElementById("my-fieldset");
console.log(fieldset.elements.length); // 3
console.log(fieldset.elements["remember-me"].value); // "on"