FormData

Example

<form>
  <label>Login:</label>
  <input type="email" name="username" placeholder="username"><br>
  <label>Password:</label>
  <input type="password" name="password" placeholder="password"><br>
</form>

const formElement = document.querySelector("form");
const formData = new FormData(formElement)
formData.get('username')

References