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In the simplest form you can use it to test if an expression evaluates to true:

{% if online == false %}
    <p>Our website is in maintenance mode. Please, come back later.</p>
{% endif %}

You can also test if an array is not empty:

{% if users %}
        {% for user in users %}
            <li>{{ user.username|e }}</li>
        {% endfor %}
{% endif %}

NOTE If you want to test if the variable is defined, use if users is defined instead.

You can also use not to check for values that evaluate to false:

{% if not user.subscribed %}
    <p>You are not subscribed to our mailing list.</p>
{% endif %}

For multiple conditions, and and or can be used:

{% if temperature > 18 and temperature < 27 %}
    <p>It's a nice day for a walk in the park.</p>
{% endif %}

For multiple branches elseif (or elif) and else can be used. You can use more complex expressions there too:

{% if product.stock > 10 %}
{% elseif product.stock > 0 %}
   Only {{ product.stock }} left!
{% else %}
{% endif %}

The rules to determine if an expression is true or false are (edge cases):

Value Boolean evaluation
empty string false
numeric zero false
whitespace-only string true
string “0” or ‘0’ true
empty table false
nil false
non-empty table true
table with __toboolean __toboolean()
cjson.null false
cbson.null() false
lyaml.null false
yaml.null false
ngx.null false
msgpack.null false

Function __toboolean() should be in the metatable.

You can add your own false-behaviour