JavaScript, AJAX & the Profiler


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Here's our next goal: write some JavaScript so that that when we click the up or down vote icons, it will make an AJAX request to our JSON endpoint. This "fakes" saving the vote to the database and returns the new vote count, which we will use to update the vote number on the page.

Adding js- Classes to the Template

The template for this page is: templates/question/show.html.twig. For each answer, we have these vote-up and vote-down links. I'm going to add a few classes to this section to help our JavaScript. On the vote-arrows element, add a js-vote-arrows class: we'll use that in JavaScript to find this element. Then, on the vote-up link, add a data attribute called data-direction="up". Do the same for the down link: data-direction="down". This will help us know which link was clicked. Finally, surround the vote number - the 6 - with a span that has another class: js-vote-total. We'll use that to find the element so we can update that number.

59 lines | templates/question/show.html.twig
// ... lines 1 - 4
{% block body %}
<div class="container">
// ... lines 7 - 36
<ul class="list-unstyled">
{% for answer in answers %}
<li class="mb-4">
<div class="d-flex justify-content-center">
// ... lines 41 - 47
<div class="vote-arrows flex-fill pt-2 js-vote-arrows" style="min-width: 90px;">
<a class="vote-up" href="#" data-direction="up"><i class="far fa-arrow-alt-circle-up"></i></a>
<a class="vote-down" href="#" data-direction="down"><i class="far fa-arrow-alt-circle-down"></i></a>
<span>+ <span class="js-vote-total">6</span></span>
{% endfor %}
{% endblock %}

Adding JavaScript inside the javascripts Block.

To keep things simple, the JavaScript code we are going to write will use jQuery. In fact, if your site uses jQuery, you probably will want to include jQuery on every page... which means that we want to add a script tag to base.html.twig. At the bottom, notice that we have a block called javascripts. Inside this block, I'm going to paste a <script> tag to bring in jQuery from a CDN. You can copy this from the code block on this page, or go to jQuery to get it.


In new Symfony projects, the javascripts block is at the top of this file - inside the <head> tag. You can keep the javascripts block up in <head> or move it down here. If you keep it up inside head, be sure to add a defer attribute to every script tag: this will cause your JavaScript to be executed after the page loads.

34 lines | templates/base.html.twig
// ... line 1
// ... lines 3 - 12
// ... lines 14 - 25
{% block javascripts %}
{% endblock %}

If you're wondering why we put this inside of the javascripts block... other than it "seems" like a logical place, I'll show you why in a minute. Because technically, if we put this after the javascripts block or before, it would make no difference right now. But putting it inside will be useful soon.

For our custom JavaScript, inside the public/ directory, create a new directory called js/. And then a new file: question_show.js.

Here's the idea: usually you will have some custom JavaScript that you want to include on every page. We don't have any right now, but if we did, I would create an app.js file and add a script tag for it in base.html.twig. Then, on certain pages, you might also need to include some page-specific JavaScript, like to power a comment-voting feature that only lives on one page.

That's what I'm doing and that's why I created a file called question_show.js: it's custom JavaScript for that page.

Inside question_show.js, I'm going to paste about 15 lines of code.

* Simple (ugly) code to handle the comment vote up/down
var $container = $('.js-vote-arrows');
$container.find('a').on('click', function(e) {
var $link = $(e.currentTarget);
url: '/comments/10/vote/'+$link.data('direction'),
method: 'POST'
}).then(function(response) {

This finds the .js-vote-arrows element - which we added here - finds any a tags inside, and registers a click listener on them. On click, we make an AJAX request to /comments/10 - the 10 is hardcoded for now - /vote/ and then we read the data-direction attribute off of the anchor element to know if this is an up vote or down vote. On success, jQuery passes us the JSON data from our endpoint. Let's rename that variable to data to be more accurate.

16 lines | public/js/question_show.js
// ... lines 1 - 4
$container.find('a').on('click', function(e) {
// ... lines 6 - 8
// ... lines 10 - 11
}).then(function(data) {

Then we use the votes field from the data - because in our controller we're returning a votes key - to update the vote total.

Overriding the javascripts Block

So... how do we include this file? If we wanted to include this on every page, it would be pretty easy: add another script tag below jQuery in base.html.twig. But we want to include this only on the show page. This is where having the jQuery script tag inside of a javascripts block is handy. Because, in a "child" template, we can override that block.

Check it out: in show.html.twig, it doesn't matter where - but let's go to the bottom, say {% block javascripts %} {% endblock %}. Inside, add a <script> tag with src="". Oh, we need to remember to use the asset() function. But... PhpStorm is suggesting js/question_show.js. Select that. Nice! It added the asset() function for us.

65 lines | templates/question/show.html.twig
// ... lines 1 - 59
{% block javascripts %}
// ... lines 61 - 62
<script src="{{ asset('js/question_show.js') }}"></script>
{% endblock %}

If we stopped now, this would literally override the javascripts block of base.html.twig. So, jQuery would not be included on the page. Instead of overriding the block, what we really want to do is add to it! In the final HTML, we want our new script tag to go right below jQuery.

How can we do this? Above our script tag, say {{ parent() }}.

65 lines | templates/question/show.html.twig
// ... lines 1 - 59
{% block javascripts %}
{{ parent() }}
<script src="{{ asset('js/question_show.js') }}"></script>
{% endblock %}

I love that! The parent() function gets the content of the parent block, and prints it.

Let's try this! Refresh and... click up. It updates! And if we hit down, we see a really low number.

AJAX Requests on the Profiler

Oh, and see this number "6" down on the web debug toolbar? This is really cool. Refresh the page. Notice that the icon is not down here. But as soon as our page makes an AJAX requests, it shows up! Yep, the web debug toolbar detects AJAX requests and lists them here. The best part is that you can use this to jump into the profiler for any of these requests! I'll right click and open this "down" vote link in a new tab.

This is the full profiler for that request in all its glory. If you use dump() somewhere in your code, the dumped variable for that AJAX requests will be here. And later, a database section will be here. This is a killer feature.

Next, let's tighten up our API endpoint: we shouldn't be able to make a GET request to it - like loading it in our browser. And... do we have anything that validates that the {direction} wildcard in the URL is either up or down but nothing else? Not yet.