HTML

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <title>მთავარი</title>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.3/jquery.min.js"></script>
    <link rel="stylesheet" type="text/css" media="screen" href="simplmb.css" />
    <script src="simplmb.js"></script>

</head>
<body>
    <form id="login" action="/login" method="POST">
        <label>
          Username:
          <input type="text" name="username" value="">
        </label>

        <label>
          Password:
          <input type="password" name="password" value="" autocomplete="off">
        </label>

        <button type="submit">Login</button>
        <br/>
        <div class="error"></div>
      </form>

      <script type="text/javascript">
      $(function() {

        var $loginForm = $('form#login');

        // handle form submit event
        $loginForm.on('submit', function(e) {
          e.preventDefault(); // prevent browser doing default post

          $.ajax({
            type: $loginForm.attr('method'), // get method from form tag
            url: $loginForm.attr('action'),  // get action url from form tag
            contentType: 'application/json', // telling to server that will send json data
            data: JSON.stringify($loginForm.serializeArray()), // sending object with fields of form

            // something went wrong
            error: function(xhr, textStatus, error) {
              try {
                var response = JSON.parse(xhr.responseText);
                if (response.message) { // if json response has message field
                  $loginForm.find('.error:first').html(response.message);
                }
              }
              catch (error) { // tried to parse response string as json, but could not - then just put error string to div
                $loginForm.find('.error:first').html(xhr.responseText);
              }
            },

            success: function(response) {
              if (response.redirectTo) {
                window.location.href = response.redirectTo;
              }
            }
          });

        });

      });
      </script>
      <a href="/reg">signup</a>
</body>
</html>