<form method="post" action="/login" id="login-form">
    <div>
      <label for="username">Username:</label>
      <input 
        type="text" 
        id="username" 
        name="username" 
      >
    </div>
    <div>
      <label for="password">Password:</label>
      <input 
        type="password" 
        id="password" 
        name="password" 
      >
    </div>
    <input type="submit" value="Login">
  </form>

  const form = document.getElementById('login-form');

  form.addEventListener('submit', (e) => {
    e.preventDefault();
    sendLoginRequest(new FormData(form));
  });
  

  
  function sendLoginRequest(formData)  
    const fetchOpts = {
      method: 'POST',
      headers: {
        Accept: 'application/json',
      },
    };

    if (navigator.credentials) {
      const credentials = new PasswordCredential({
        id: formData.get('username'),
        password: formData.get('password'),
      });

      credentials.additionalData = formData;
      fetchOpts.credentials = credentials;
    } else {
      fetchOpts.body = formData;
    }

    return fetch('/api/login', fetchOpts)
    .then((res) => res.json())
    .then((json) => validateResponse(json, formData))
    .then((redirectUrl) => {
      window.location = redirectUrl;
    });
  }

  
  // Validate the response from the auth service. Returns a URL to redirect
  // the user to.
  function validateResponse(res, credentials)  
    if (res.valid) {
      // Logged in successfully. If the browser supports the Credential
      // Management API we can attempt to save the provided login details.
      if (navigator.credentials) {
        return navigator.credentials.store(credentials)
        .then(() => res.redirectUrl);
      }

      return res.redirectUrl;
    }

    throw new Error('Login failed.');
  }

  // If the browser supports the Credential Management API we can attempt to
  // get a stored password credential. If no credential is stored, of if the
  // call fails for any reason, the user can still log in like normal.
  if (navigator.credentials) {
    navigator.credentials.get({
      password: true,
    })
    .then((credentials) => {
      if (credentials) {
        return sendLoginRequest(credentials);
      }

      return null;
    });
  }

  
  if (navigator.credentials) {
    navigator.credentials.requireUserMediation()
    .then(() => {
      // Redirect to login page.
    });
  }