Majestix: 1 revision imported: Initial Setup

2026-04-12T10:04:52Z

1 revision imported: Initial Setup

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<languages />
{{PD Help Page}}
{{Magic words}}
<translate> When applying [[<tvar name=1>Special:MyLanguage/Help:Extension:ParserFunctions</tvar>|ParserFunctions]] to [[<tvar name=2>Special:MyLanguage/Help:Templates</tvar>|template]] parameters, a pipe symbol ("|") may be used to provide a default value, which is used when a parameter is not defined.</translate>
<translate> Used in an #if parser function, the unexpanded text from the undefined parameter will evaluate as true, which may be an unexpected result.</translate>

{| class="wikitable" width="90%"
|+ <translate> Testing parameters with and without default values</translate>
|-
! colspan=2 | <translate> Parameter</translate>
! 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>''<<translate> parameter</translate>>''<nowiki>|True|False}}</nowiki></kbd>
|-
! <translate> Description</translate>
! <translate> Example, unnamed and named</translate>
! <kbd><nowiki>{{{1}}}</nowiki></kbd>, <kbd><nowiki>{{{param}}}</nowiki></kbd>
! <kbd><nowiki>{{{1|}}}</nowiki></kbd>, <kbd><nowiki>{{{param|}}}</nowiki></kbd>
|-
| <translate> Undefined. More appropriate for use in named parameters.</translate>
|<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''
|-
| <translate> Defined, but empty or null.</translate>
| <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''
|-
| <translate> Defined, non-empty, and non-null.</translate>
| <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>

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

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

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

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

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

<translate>

The second usage (<tvar name=1><code><nowiki>{{{1|}}}</nowiki></code></tvar>, 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 == 
</translate>
<translate> To distinguish a possibly empty parameter from an unspecified one, compare it to itself using <tvar name=ifeq><code><nowiki>{{#ifeq:}}</nowiki></code></tvar> and ''different defaults''.</translate>
<translate> What the defaults are does not matter as long as they are different, so they are typically chosen to be short.</translate>
<translate> The following all work equivalently:</translate>

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

<translate>

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):
</translate>

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

<translate>

<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''.
</translate>

<translate> 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''.</translate>
<translate> The suffixed parameter name must be meaningless to the <code>wrapped_template</code> for this to work properly.</translate>

<translate>
==See also== 
</translate>
* {{ll|Help:Extension:ParserFunctions}}
* {{ll|Help:Templates}}
* {{ll|Manual:Template limits#Expensive parser function calls}}
<translate>

*[[<tvar name=ext>Special:MyLanguage/Extension:Loops##forargs (experimental)</tvar>|Extension:Loops#forargs]]
</translate>

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

Help:Parser functions in templates - Revision history

Majestix: 1 revision imported: Initial Setup

en>Tactica: Dark mode fixes.