Tests whether a value is undefined, returning the given argument.

Whereas the |default filter also falls back to the default value when a value is an empty string or false, this filter only falls back to its value when the input value is the Erlang undefined atom.

This can be used for setting values which default to true if they are never set.

For example:

{% if value|if_undefined:true %}The value is true or undefined{% endif %}

If the value is undefined, the output will be “The value is true or undefined”.