How to check for null in Twig?

What construct should I use to check whether a value is NULL in a Twig template?

Depending on what exactly you need:

• is null checks whether the value is null:

{% if var is null %}
    {# do something #}
{% endif %}

• is defined checks whether the variable is defined:

{% if var is not defined %}
    {# do something #}
{% endif %}

Additionally the is sameas test, which does a type strict comparison of two values, might be of interest for checking values other than null (like false):

{% if var is sameas(false) %}
    {# do something %}
{% endif %}