Help:Parser functions in templates

When applying ext>Special:MyLanguage/Help:Extension:ParserFunctions|ParserFunctions to templates>Special:MyLanguage/Help:Templates|template parameters, a pipe symbol ("|") may be used to provide a default value, which is used when a parameter is not defined. Used in an #if parser function, the unexpanded text from the undefined parameter will evaluate as true, which may be an unexpected result.




 * Sample A:


 * Result:




 * Sample B:


 * Result:

The second usage (, sample B) with present empty default is often the desired way to handle situations where a parameter exists, but is comprised only of empty space.

Testing and passing undefined parameters
To distinguish a possibly empty parameter from an unspecified one, compare it to itself using   and different defaults. What the defaults are does not matter as long as they are different, so they are typically chosen to be short. The following all work equivalently:



In rare cases, a template behaves differently when a parameter is unspecified compared to when it is specified but empty. When this template is used by a wrapper template (which uses the same set of named parameters), one way to ensure undefined parameters remain undefined is as follows (the technique also works with numbered parameters):





receives a defined normal_parameter in all cases. When normal_parameter is defined but empty and when it is undefined,  receives an empty normal_parameter.

By contrast, the  receives a defined sensitive_parameter only when it is indeed defined; when sensitive_parameter is undefined, the   changes the parameter name to sensitive_parameterNULL. The suffixed parameter name must be meaningless to the  for this to work properly.