defined
The defined test checks to see if a variable is defined in the current context.
Examples
Test to see if a variable is defined.
{% if foo is defined %}
...
{% endif %}
Test a variable attribute or an array key value
{% if foo.bar is defined %}
...
{% endif %}
{% if foo['bar'] is defined %}
...
{% endif %}
Negative test
{% if foo is not defined %}
...
{% endif %}