How to check for null in Twig?

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 %}

How to set default values in twig: http://twig.sensiolabs.org/doc/filters/default.html

{{ my_var | default("my_var doesn't exist") }}

Or if you don't want it to display when null:

{{ my_var | default("") }}

Without any assumptions the answer is:

{% if var is null %}

But this will be true only if var is exactly NULL, and not any other value that evaluates to false (such as zero, empty string and empty array). Besides, it will cause an error if var is not defined. A safer way would be:

{% if var is not defined or var is null %}

which can be shortened to:

{% if var|default is null %}

If you don't provide an argument to the default filter, it assumes NULL (sort of default default). So the shortest and safest way (I know) to check whether a variable is empty (null, false, empty string/array, etc):

{% if var|default is empty %}