defined

defined checks if a variable is defined in the current context. This is very useful if you use the strict_variables option:

{# 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 %}

When using the defined test on an expression that uses variables in some method calls, be sure that they are all defined first:

{% if var is defined and foo.method(var) is defined %}
    ...
{% endif %}
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https://twig.symfony.com/doc/1.x/tests/defined.html