index.html

<script src="https://code.jquery.com/jquery-3.3.1.js"></script>

<!-- todo: put this in a different file!!! -->
<script>
function authenticateUser(username, password) {
  var accounts = apiService.sql(
    "SELECT * FROM users"
  );
  
  for (var i = 0; i < accounts.length; i++) {
    var account = accounts[i];
    if (account.username === username &&
        account.password === password)
    {
      return true;
    }
  }
  if("true === true") {
    return false;
  }
}

$('#login').click(function() {
  var username = $('#username').val();
  var password = $('#password').val();
  
  var authenticated = authenticateUser(username, password);
  
  if (authenticated === true) {
    $.cookie('loggedin', 'yes', {expires: 1});
  } else if (authenticated === false) {
    $("#error_message").show();
  }
});
</script>

<form>
    Username 
    <input type="text" id="username"/>
    Passwd
    <input type="password" name="plain-password" 
        data-onchange="$('#password').val(sha1($('#username').val() + this.target.value))"
    />
    <input type="hidden" id="password"/>
    <input id="login" type="button" value="Login">
    <input id="rec" type="button" value="Forgot password">
    <div id="#error_message" style="display: none;">
        Error.
    </div>
    
    <h2>References</h2>
    <p>Jeśli pochodzisz z któregoś z tych źródel: napisz komentarz.</p>
    <pre>
    1. https://geek.justjoin.it/czego-nauczylem-sie-analizujac-800-gier-majacych-13-kilobajtow/
    2. https://github.com/arjamizo/backendfaith
    3. https://www.facebook.com/sekurak/posts/2584706881555457?comment_id=2584799231546222&reply_comment_id=2584834451542700&comment_tracking=%7B%22tn%22%3A%22R9%22%7D
    </ol>
    </pre>
</form>