Template:

  {% if value_json.name | first is defined %}a{% else %}b{% endif %}

With binding:

{
  "name": [ "c" ]
}

raises com.hubspot.jinjava.interpret.FatalTemplateErrorsException: Error parsing 'value_json.name | first is defined': syntax error at position 27, encountered 'is', expected '}'

This works in Python Jinja. I can work around with parentheses:

{% if (value_json.name | first) is defined %}a{% else %}b{% endif %}