twig is defined code example
Example 1: twig is set variable
{# defined works with variable names #}
{% if foo is defined %}
...
{% endif %}
{# and attributes on variables names #}
{% if foo.bar is defined %}
...
{% endif %}
{% if foo['bar'] is defined %}
...
{% endif %}
Example 2: twig variable is defined
{% if varname is defined %}
Variable exists.
{% endif %}