defined

defined checks if a variable is defined in the current context.

{# 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 %}