lUser L

<form id="book_form" method="POST" action="/submit/book">
        <div id="message">
            <?php if (!empty($_SESSION['messages'])) echo $_SESSION['messages'];?>
        </div>
        <div>
            <label for="title">Title</label>
            <input type="text" name="title" id="title" class="u-full-width" autocomplete="off" value="<?php if (!empty($_SESSION['title'])) echo $_SESSION['title'];?>">
        </div>
        <div>
            <label for="author">Author</label>
            <input type="text" name="author" id="author" class="u-full-width" autocomplete="off" value="<?php if (!empty($_SESSION['author'])) echo $_SESSION['author'];?>">
        </div>
        <div>
            <label for="isbn">ISBN#</label>
            <input type="text" name="isbn" id="isbn" class="u-full-width" autocomplete="off" value="<?php if (!empty($_SESSION['isbn'])) echo $_SESSION['isbn'];?>">
        </div>
        <div>
            <input type="submit" name="submit" value="Submit" class="u-full-width">
        </div>
    </form>

document.getElementById('book_form').addEventListener('submit', function(event)
	{
		var title  = document.getElementById('title').value,
			author = document.getElementById('author').value,
			isbn   = document.getElementById('isbn').value;

		title  = title.trim();
		author = author.trim();
		isbn   = isbn.trim();

		var params = 'title=' + title + '&author=' + author + '&isbn=' + isbn + '&submit=Submit';

		getAjax('POST', '/submit/book', params, getShowAlert);

		event.preventDefault();
	}, false);

function getAjax(method, uri, params, action)
{
	xhr.open(method, uri, true);

	if (xhr && method === 'GET')
	{
		xhr.setRequestHeader('X-Requested-With', 'XMLHttpRequest');
	}

	else if (xhr && method === 'POST')
	{
		xhr.setRequestHeader('X-Requested-With', 'XMLHttpRequest');
		xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
	}

	xhr.onreadystatechange = action;

	xhr.send(params);
}

var getShowAlert = function()
	{
		if (xhr.readyState === 4)
		{
			if (xhr.status === 200)
			{
				var mes = JSON.parse(xhr.responseText);

				if (mes.message.error)
				{
					showAlert(mes.message.error, 'error');
				}

				else if(mes.message.success)
				{
					getAjax('GET', '/', null, getContent);
					showAlert(mes.message.success, 'success');
					clearFields();
				}
			}

			else console.log(xhr.status + ' (' + xhr.statusText + ')');
		}
	}

function isAjax() { return (isset($_SERVER['HTTP_X_REQUESTED_WITH']) && !empty($_SERVER['HTTP_X_REQUESTED_WITH'])) ? true : false;}

    $uri = $_SERVER['REQUEST_URI'];
    
   else if ($uri === '/submit/book')
    {
        if (isAjax())
        {
            if ($_SERVER['REQUEST_METHOD'] === 'POST')
            {
                if (isset($_POST['submit']) && !empty($_POST['submit']))
                {
                    $title  = htmlspecialchars(trim($_POST['title']));
                    $author = htmlspecialchars(trim($_POST['author']));
                    $isbn   = htmlspecialchars(trim($_POST['isbn']));

                    if (empty($title) || empty($author) || empty($isbn)) echo '{"message":{"error":"Please fill in all fields"}}';

                    else
                    {
                        if ($models->insertItem($title, $author, $isbn)) echo '{"message":{"success":"Book item create!"}}';
                        else echo '{"message":{"error":"Book item no create!"}}';
                    }
                }
            }
        }

        else
        {
            if ($_SERVER['REQUEST_METHOD'] === 'POST')
            {
                if (isset($_POST['submit']) && !empty($_POST['submit']))
                {
                    $title  = htmlspecialchars(trim($_POST['title']));
                    $author = htmlspecialchars(trim($_POST['author']));
                    $isbn   = htmlspecialchars(trim($_POST['isbn']));

                    if (empty($title) || empty($author) || empty($isbn))
                    {
                        $_SESSION['title']  = $title;
                        $_SESSION['author'] = $author;
                        $_SESSION['isbn']   = $isbn;
                        $_SESSION['messages'] = '<div class="error">Please fill in all fields</div>';

                        header('Location: /');
                        exit;
                    }

                    else
                    {
                        if ($models->insertItem($title, $author, $isbn))
                        {
                            $_SESSION['messages'] = '<div class="success">Book Added!</div>';

                            header('Location: /');
                            exit;
                        }
                    }
                }
            }
        }
    }