I have created a Login-form, basically like so:

 <form class="form" id="login" th:action="@{/login}" method="post">
     <input type="text" class="form__input" id="username" name="username" autofocus placeholder="Username or eMail" />
     <input type="password" class="form__input" id="password" name="password" placeholder="Password" />
     <button class="form__button" type="submit">Login</button>
 </form>

I have attached a js file to this html page, in which I add the following listeners:

const form = document.querySelector("#login");
const submitButton = document.querySelector("button");
const usernameInputField = document.querySelector("#username");
const passwordInputField = document.querySelector("#password");

 form.addEventListener("submit", e => {
    checkInputValidity("user");
    checkInputValidity("password");

    if (!isInputValid()) {
      e.preventDefault();
    }
  });

  passwordInputField.addEventListener("keyup", e => {
    if (e.keyCode === 13) {
      submitButton.focus();
      form.submit();
    }
  })

I think important is the checkInputValidity() method:

  function checkInputValidity(type) {
    switch (type) {
      case "user":
        isEmpty(usernameInputField.value) ?
          isUsernameValid = false :
          isUsernameValid = true;
        break;
      case "password":
        isEmpty(passwordInputField.value) ?
          isPasswordValid = false :
          isPasswordValid = true;
        break;
    }
  }

isInputValid() simply checks if either of the input fields is empty and then returns false, else returns true:

  function isInputValid() {
    return !!(isUsernameValid && isPasswordValid);
  }

Here is the problem:

After I have entered the correct login data, when clicking on the Login-button, I have successfully logged in. However, when entering the correct login data and hitting the 'Enter' button, I am just redirected to my Login-page. This seems to be related to the checkInputValidity() method as removing this method from my form-submit-listener reverts the issue: Now, when clicking on the Login-button, nothing happens. But when hitting the 'Enter' button, I am successfully logged in.

This whole behavior is very strange to me. Can somebody help me out?