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<languages />
{{PD Help Page}}
{{Magic words}}
When applying [[Special:MyLanguage/Help:Extension:ParserFunctions|ParserFunctions]] to [[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.

{| class="wikitable" width="90%"
|+ Testing parameters with and without default values
|-
! colspan=2 | Parameter
! rowspan=2 | <kbd><nowiki>{{{1}}}</nowiki>, <nowiki>{{{param}}}</nowiki></kbd>
! rowspan=2 | <kbd><nowiki>{{{1|}}}</nowiki>, <nowiki>{{{param|}}}</nowiki></kbd>
! colspan=2 | <kbd><nowiki>{{#if:</nowiki>''<parameter>''<nowiki>|True|False}}</nowiki></kbd>
|-
! Description
! Example, unnamed and named
! <kbd><nowiki>{{{1}}}</nowiki></kbd>, <kbd><nowiki>{{{param}}}</nowiki></kbd>
! <kbd><nowiki>{{{1|}}}</nowiki></kbd>, <kbd><nowiki>{{{param|}}}</nowiki></kbd>
|-
| Undefined. More appropriate for use in named parameters.
|<code><nowiki>{{template}}</nowiki></code>
| <code><nowiki>{{{1}}}</nowiki></code>
|
| style="text-align:center; color:var(--color-base, #202122); background-color:var(--background-color-success-subtle, #dff2eb)" | '''True'''
| style="text-align:center; color:var(--color-base, #202122); background-color:var(--background-color-error-subtle--active, #ffc8bd)" | ''False''
|-
| Defined, but empty or null.
| <code><nowiki>{{template|}}</nowiki></code>, <code><nowiki>{{template|1=}}</nowiki></code>, <code><nowiki>{{template|param=}}</nowiki></code>
|
|
| style="text-align:center; color:var(--color-base, #202122); background-color:var(--background-color-error-subtle--active, #ffc8bd)" | ''False''
| style="text-align:center; color:var(--color-base, #202122); background-color:var(--background-color-error-subtle--active, #ffc8bd)" | ''False''
|-
| Defined, non-empty, and non-null.
| <code><nowiki>{{template|value}}</nowiki></code>, <code><nowiki>{{template|1=value}}</nowiki></code>, <code><nowiki>{{template|param=value}}</nowiki></code>
| value
| value
| style="text-align:center; color:var(--color-base, #202122); background-color:var(--background-color-success-subtle, #dff2eb)" | '''True'''
| style="text-align:center; color:var(--color-base, #202122); background-color:var(--background-color-success-subtle, #dff2eb)" | '''True'''
|}

*<code><nowiki>{{{1}}}</nowiki></code>

:;Sample A: <code><nowiki>{{#if: {{{1}}}</nowiki> | Parameter 1 is not defined, or is defined and non-null/non-empty. | Parameter 1 is null. It contains only empty string(s) or breaking space(s) etc.<nowiki>}}</nowiki></code>

:;Result: <code>{{#if: {{{1}}} | Parameter 1 is not defined, or is defined and non-null/non-empty. | Parameter 1 is null. It contains only empty string(s) or breaking space(s) etc.}}</code>

*<code><nowiki>{{{1|}}}</nowiki></code>

:;Sample B: <code><nowiki>{{#if: {{{1|}}}</nowiki> | Parameter 1 is defined and non-null/non-empty. | Parameter 1 is not defined, or is defined but null. It contains only empty string(s) or breaking space(s) etc.<nowiki>}}</nowiki></code>

:;Result: <code>{{#if: {{{1|}}} | Parameter 1 is defined and non-null/non-empty. | Parameter 1 is not defined, or is defined but null. It contains only empty string(s) or breaking space(s) etc.}}</code>

The second usage (<code><nowiki>{{{1|}}}</nowiki></code>, 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 <code><nowiki>{{#ifeq:}}</nowiki></code> 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:

:<code><nowiki>{{#ifeq:{{{v|+}}}|{{{v|-}}}|</nowiki> v was defined (and may be empty) | v was not defined <nowiki>}}</nowiki></code>
:<code><nowiki>{{#ifeq:{{{v|}}}|{{{v|-}}}|</nowiki> v was defined (and may be empty) | v was not defined <nowiki>}}</nowiki></code>
:<code><nowiki>{{#ifeq:{{{v|}}}|{{{v}}}|</nowiki> v was defined (and may be empty) | v was not defined <nowiki>}}</nowiki></code>

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):

<dl><dd><pre>
{{wrapped_template|
normal_parameter={{{normal_parameter|}}}|
sensitive_parameter{{#if:{{{sensitive_parameter|}}}||NULL}}={{{sensitive_parameter}}}
}}
</pre></dd></dl>

<code>wrapped_template</code> receives a defined ''normal_parameter'' in all cases. When ''normal_parameter'' is defined but empty and when it is undefined, <code>wrapped_template</code> receives an empty ''normal_parameter''.

By contrast, the <code>wrapped_template</code> receives a defined ''sensitive_parameter'' ''only'' when it is indeed defined; when ''sensitive_parameter'' is undefined, the <code>#if</code> changes the parameter name to ''sensitive_parameterNULL''.
The suffixed parameter name must be meaningless to the <code>wrapped_template</code> for this to work properly.

==See also==
* {{ll|Help:Extension:ParserFunctions}}
* {{ll|Help:Templates}}
* {{ll|Manual:Template limits#Expensive parser function calls}}
*[[Special:MyLanguage/Extension:Loops##forargs (experimental)|Extension:Loops#forargs]]

[[Category:Extension help{{#translation:}}|Parser functions in templates]]

Help:Parser functions in templates/en - Revision history

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