If Statements with "Tests"

But when we try the contact page, it blows up

Uncaught exception Twig_Error_Runtime with message "Variable backgroundColor does not exist in _banner.twig at line 5"

We're not passing a backgroundColor in the include call in contact.twig, so Twig gets angry! We can of course pass this variable to the template. But instead, let's check to see if the variable is defined in the _banner.twig template, and default to lightblue if it is not. To do this, we can use an if statement and a special defined test.

{# templates/_banner.twig #}

{% if backgroundColor is defined %}
    {# ... do something here #}
{% endif %}

Let's also see another new Twig tag called set, which sets a new variable.

{# templates/_banner.twig #}

{% if backgroundColor is defined %}
    {% set backgroundColor = backgroundColor %}
{% else %}
    {% set backgroundColor = 'lightblue' %}
{% endif %}

<div class="well" style="background-color: {{ backgroundColor }};">
    <h3>Save some Krill!</h3>

    <p>Sale in summer suits all this weekend! {{ pageData.title }}</p>
</div>

Try out both pages in the browser. Awesomesauce!

Let's look again at the if background is defined. Normally an if statement says something like if foo != 'bar' or if isPublished == true. That all works totally fine in Twig.

But in addition to == and != and the others, you can say is and follow that by a word like defined, even, odd, empty or several other words. These are called tests, and they're listed once again right back on the main Documentation page of the Twig website.

For example, instead of using the length filter and seeing if the number of items in the products collection is zero, we could say if products is empty:

{# template/homepage.twig #}
{# ... #}

{% if products is empty %}
    <div class="alert alert-error span12">
        It looks like we're out of really awesome-looking penguin clothes :/.
    </div>
{% endif %}

If for some reason we wanted to know if the total number of products were even, we could use the length filter to get the number, then check that the number is "even" by using the even test:

{% if products|length is even %}
    <p>
        There are an even number of products
    </p>
{% endif %}

The tests are easy to use and can shorten the code needed to do some things, so don't forget about them!

Negating a Test

You can also negate a test by using the not keyword. We can use this to simplify our code from earlier:

{# templates/_banner.twig #}

{% if backgroundColor is not defined %}
    {% set backgroundColor = 'lightblue' %}
{% endif %}

<div class="well" style="background-color: {{ backgroundColor }};">
    <h3>Save some Krill!</h3>

    <p>Sale in summer suits all this weekend! {{ pageData.title }}</p>
</div>

Awesome! At this point, you know a lot of tools in Twig. Let's keep going and learn some more.