AJAX with jQuery
================

`jQuery`_ is a small JavaScript library commonly used to simplify working
with the DOM and JavaScript in general.  It is the perfect tool to make
web applications more dynamic by exchanging JSON between server and
client.

JSON itself is a very lightweight transport format, very similar to how
Python primitives (numbers, strings, dicts and lists) look like which is
widely supported and very easy to parse.  It became popular a few years
ago and quickly replaced XML as transport format in web applications.

.. _jQuery: https://jquery.com/

Loading jQuery
--------------

In order to use jQuery, you have to download it first and place it in the
static folder of your application and then ensure it's loaded.  Ideally
you have a layout template that is used for all pages where you just have
to add a script statement to the bottom of your ``<body>`` to load jQuery:

.. sourcecode:: html

   <script src="{{ url_for('static', filename='jquery.js') }}"></script>

Another method is using Google's `AJAX Libraries API
<https://developers.google.com/speed/libraries/>`_ to load jQuery:

.. sourcecode:: html

    <script src="//ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
    <script>window.jQuery || document.write('<script src="{{
      url_for('static', filename='jquery.js') }}">\x3C/script>')</script>

In this case you have to put jQuery into your static folder as a fallback, but it will
first try to load it directly from Google. This has the advantage that your
website will probably load faster for users if they went to at least one
other website before using the same jQuery version from Google because it
will already be in the browser cache.

Where is My Site?
-----------------

Do you know where your application is?  If you are developing the answer
is quite simple: it's on localhost port something and directly on the root
of that server.  But what if you later decide to move your application to
a different location?  For example to ``http://example.com/myapp``?  On
the server side this never was a problem because we were using the handy
:func:`~flask.url_for` function that could answer that question for
us, but if we are using jQuery we should not hardcode the path to
the application but make that dynamic, so how can we do that?

A simple method would be to add a script tag to our page that sets a
global variable to the prefix to the root of the application.  Something
like this:

.. sourcecode:: html+jinja

   <script>
     $SCRIPT_ROOT = {{ request.script_root|tojson }};
   </script>


JSON View Functions
-------------------

Now let's create a server side function that accepts two URL arguments of
numbers which should be added together and then sent back to the
application in a JSON object.  This is a really ridiculous example and is
something you usually would do on the client side alone, but a simple
example that shows how you would use jQuery and Flask nonetheless::

    from flask import Flask, jsonify, render_template, request
    app = Flask(__name__)

    @app.route('/_add_numbers')
    def add_numbers():
        a = request.args.get('a', 0, type=int)
        b = request.args.get('b', 0, type=int)
        return jsonify(result=a + b)

    @app.route('/')
    def index():
        return render_template('index.html')

As you can see I also added an `index` method here that renders a
template.  This template will load jQuery as above and have a little form where
we can add two numbers and a link to trigger the function on the server
side.

Note that we are using the :meth:`~werkzeug.datastructures.MultiDict.get` method here
which will never fail.  If the key is missing a default value (here ``0``)
is returned.  Furthermore it can convert values to a specific type (like
in our case `int`).  This is especially handy for code that is
triggered by a script (APIs, JavaScript etc.) because you don't need
special error reporting in that case.

The HTML
--------

Your index.html template either has to extend a :file:`layout.html` template with
jQuery loaded and the `$SCRIPT_ROOT` variable set, or do that on the top.
Here's the HTML code needed for our little application (:file:`index.html`).
Notice that we also drop the script directly into the HTML here.  It is
usually a better idea to have that in a separate script file:

.. sourcecode:: html

    <script>
      $(function() {
        $('a#calculate').bind('click', function() {
          $.getJSON($SCRIPT_ROOT + '/_add_numbers', {
            a: $('input[name="a"]').val(),
            b: $('input[name="b"]').val()
          }, function(data) {
            $("#result").text(data.result);
          });
          return false;
        });
      });
    </script>
    <h1>jQuery Example</h1>
    <p><input type=text size=5 name=a> +
       <input type=text size=5 name=b> =
       <span id=result>?</span>
    <p><a href=# id=calculate>calculate server side</a>

I won't go into detail here about how jQuery works, just a very quick
explanation of the little bit of code above:

1. ``$(function() { ... })`` specifies code that should run once the
   browser is done loading the basic parts of the page.
2. ``$('selector')`` selects an element and lets you operate on it.
3. ``element.bind('event', func)`` specifies a function that should run
   when the user clicked on the element.  If that function returns
   `false`, the default behavior will not kick in (in this case, navigate
   to the `#` URL).
4. ``$.getJSON(url, data, func)`` sends a ``GET`` request to `url` and will
   send the contents of the `data` object as query parameters.  Once the
   data arrived, it will call the given function with the return value as
   argument.  Note that we can use the `$SCRIPT_ROOT` variable here that
   we set earlier.

Check out the :gh:`example source <examples/javascript>` for a full
application demonstrating the code on this page, as well as the same
thing using ``XMLHttpRequest`` and ``fetch``.
