View source for Help:Template

{{short description|Main technical help page on templates}}
{{for|lists of commonly used templates|WP:Template index}}
{{For|a quick introduction to templates|Help:A quick guide to templates}}
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A '''template''' is a Wikipedia page created to be included in other pages. Templates usually contain repetitive material that might need to show up on a larger number of articles or pages. They are commonly used for [[WP:Glossary#Boilerplate text|boilerplate]] messages, standardized warnings or notices, [[Help:Infobox|infoboxes]], [[WP:Navbox|navigational boxes]], and similar purposes.  Templates can have parameters so that the same page generates different text on multiple pages that include it.

The most common method of inclusion is called [[Help:Transclusion|transclusion]], where the wiki source of the target page contains a reference to the template, using the {{tnull|{{var|Template&nbsp;name}}}} syntax. Another method is [[Help:Substitution|substitution]], where the content of the template is copied into the wiki source of the target page, just once, when it is saved.

[[Help:A quick guide to templates]] gives a brief introduction to the subject. There is further help from MediaWiki and Wikimedia at [[mw:Help:Templates]], [[m:Help:Template]], and [[m:Help:Advanced templates]].

==General description==
[[File:Introduction to templates on Wikipedia.webm|thumb|A basic overview of how templates work (8-minute video)]]

Most templates are in the [[WP:Template namespace|template namespace]], which means that they have titles in the form "Template:{{var|XXXX}}". It is possible, however, to transclude and substitute from any namespace,{{efn|Namespaces from which transclusion is not allowed are specified on a wiki by the variable <code>[[mw:Manual:$wgNonincludableNamespaces|$wgNonincludableNamespaces]]</code>.}} and so some template pages are placed in other namespaces, such as the [[WP:User pages|user namespace]]. Template pages have associated [[Help:Talk pages|talk pages]].

Templates can contain any desired [[Help:Wikitext|wikitext]], including calls to other templates. There is some programming ability: customizable values (via [[H:PARAMETER|parameters]]); calculation and branchings (using [[Help:Parser functions|parser functions]]); and access to wiki-specific variables ([[Help:Magic words|magic words]]), such as dates, times, and page names. They may also contain markup to define certain parts of the page to be included while other parts are not. This means that when you display the template page itself, you may see more than when you display a page that includes it (for example, it can contain documentation, categories, etc. for the template).

To call a template (cause it to be [[Help:Transclusion|transcluded]] or [[Help:Substitution|substituted]] in the page), a page contains wikitext in double braces.  For example, the wikitext {{tnull|under construction}} calls the {{tl|under construction}} template.  Sometimes the call includes parameters and looks like <code><nowiki>{{under construction|comment=This is a comment}}</nowiki></code>.  Note that template calls are not the only thing for which double braces are used in wikitext.

Note that while templates are the usual way transclusion and substitution happen on Wikipedia, they are not the only way.

==Using templates==
===General===
Using a template is much like calling a [[Subroutine|function]] in a programming language – call it, and it returns a value. For a template, the return value is called the ''expansion'' of the template and is treated as part of the calling page source.  Like functions, some templates accept parameters that affect the output.

Calling a template causes it to be either transcluded or substituted in the page that calls it (which is known as the ''target page'').

Transcluding a template means that when MediaWiki displays the page, it treats the template as if it were in the wiki source of the page, whereas substituting means MediaWiki does that when it <em>saves</em> the page (i.e. the content of the template gets stored as part of the target page and subsequent editors cannot tell that it got there via a template).

To transclude a template into a page, put {{tnull|{{var|Template&nbsp;name}}}} in the page source at the place where the template is to appear. The first letter of each word of the title may equivalently be lower- or upper-case.

To substitute a template into a page, use {{tnull|subst:{{var|Template&nbsp;name}}}} instead.

This wikitext is called a ''template call''.

In [[MediaWiki]], the [[wiki software]] that Wikipedia uses, [[Help:Magic words#Variables|variables]] are distinct from templates, but they are both identified by double [[Curly brackets|braces]] <code><nowiki>{{ }}</nowiki></code> and they both return a value.

Whereas MediaWiki variable names are all uppercase, template names have the same basic features and [[WP:NCHASHTAG|limitations]] as all [[Help:Page name|page names]]: they are [[Case sensitivity|case-sensitive]] (except for the first character); [[underscore]]s are parsed as spaces; and they cannot contain any of these characters: <code><nowiki># < > [ ] | { }</nowiki></code>. This is because those are reserved for [[wiki markup]].

The [[number sign]] <code>#</code> is called a [[URI fragment|fragment identifier]] because it denotes a {{em|fragment}} or section of a document (such as a [[Help:Section|section]] in a Wikipedia article). Although it [[WP:ANCHOR|can be used]] to link to a section of a template page (like [[Template:Portal#Example]]), there is no reason to put a fragment identifier or fragment name in a template reference. In {{tnull|Portal{{var|#Location}}|Books}}, for example, the string <code>{{var|#Location}}</code> has no purpose and is thus ignored.

The template namespace is the default, so you can leave out the namespace <code>Template:</code> in the template name, and it is conventional to do so. However, you must specify the namspace prefix for templates in other namespaces, such as <code>User:</code>. To transclude a page in [[WP:MAINSPACE|mainspace]], precede its title with a colon, as <code>{<nowiki />{:{{var|Page name}}}}</code>.

{{A note}} Attempting to transclude a template that does not exist produces a [[WP:Red link|red link]], just like linking to any other nonexistent page. Following the link allows one to create that particular template. It is not possible to transclude pages between projects (such as different-language Wikipedias or MediaWiki)—to use a template on another language project, a copy of the template must be created in that project.

===Parameters===
<!--[[Help:Parameters]] redirects here, so: -->
{{about|template parameters|search parameters|Help:Searching#Parameters|section=yes}}
{{shortcut|H:PARAMETER|WP:PARAMETER}}

The basic transclusion syntax given above can be extended with [[parameter]]s, which are used to control the template's output. The syntax for this is
{{tnull|{{var|Template name}}|{{var|parameter}}|{{var|parameter}}|{{var|...}}}}
where <code>{{var|Template name}}</code> is the name of the template, and each <code>{{var|parameter}}</code> may either contain just a value (these are called ''{{dfn|unnamed parameters}}'') or be of the form <code>{{var|name}}={{var|value}}</code> (''{{dfn|named parameters}}''). The first, second, third, etc. unnamed parameters are given the names <code>1</code>, <code>2</code>, <code>3</code>, etc.

Whitespace characters (spaces, tabs, returns) are stripped from the beginnings and ends of {{em|named}} parameter names and values, but not from the middle: thus <code><nowiki>{{ ... | myparam = this is a test }}</nowiki></code> has the same effect as <code><nowiki>{{ ... |myparam=this is a test}}</nowiki></code>. This does not apply to {{em|unnamed}} parameters, where all whitespace characters are preserved.

What parameters (if any) can or should be passed to a template and how they are to be named is defined in the coding of that template. A template call can specify named parameters in any order. Superfluous or misnamed parameters are ignored; unspecified parameters are assigned default values. If a parameter is specified more than once, the last value takes effect.

The value of a parameter can be the empty string, such as when the pipe or equals sign is followed immediately by the next pipe or the closing braces. This is different from not specifying the parameter at all, which results in a default value, although templates are often coded so as to behave the same in both cases.

If a parameter call specifies a parameter which is not defined in the template, it has no effect. Sometimes editors do this on purpose:  For example, {{para|reason}} is frequently used as a pseudo-parameter to explain briefly in the wiki source why the template call was put there.{{efn|Some templates, such as {{tlx|Requested move}}, have code to display {{para|reason}} as visible output; whether to do so is determined on a template-by-template basis.}} Some templates use {{ml|check for unknown parameters|check|...}} to warn the editor if a parameter is being used that is not accounted for in the template's code; this is mostly used for [[Help:Infobox|infoboxes]] and other templates with a large number of complicated parameters, where the presence of an unknown one is usually an unintentional error. If you update such a template to define a new parameter, its call to the module must also be updated to include the new parameter.

Wikitext syntax does not allow natively to create truly [[variadic template]]s, but only pseudo-variadic ones, that check the incoming parameters one by one until a certain fixed amount. It is possible to break this limitation however by using dedicated modules. For simple cases, {{ml|separated entries|main}} allows to expand all sequential parameters blindly and has the ability to set custom delimiters. For more complex cases, {{ml|params|}} allows to count, list, map, filter and propagate all incoming parameters without knowing their number in advance.

===Examples===

{{A note}} If you wish to experiment with any of these, you can use the [[Template:Template sandbox|template sandbox]] or your user page or sandbox.  You can try out your template with [[Special:ExpandTemplates]].

An example of a very simple template can be found at [[Template:TM]], which expands to wikitext that places the trademark symbol (™), linked to the [[trademark]] article, at that point in the displayed page. A programmer would say that the template "returns" the trademark symbol link wikitext.

Display [[Template:TM]] (by clicking on the foregoing link), then click on the "Edit" tab to see the template code (its page source). The active part of that code, which becomes the expansion of the template when it is transcluded, is the single link wikitext <code><nowiki>[[Trademark|™]]</nowiki></code>. The remainder of the page source is enclosed between {{xtag|noinclude}} tags, so it shows up when you display the template page itself but not when you display a page that calls the template.

To transclude Template:TM onto another page (i.e., to use it on another page), type {{tlx|TM}} into the source of that page (the target page) and click {{Button|Show preview}}. The page will be displayed with the template call replaced by the expansion of the template, as if the wikitext actually contained <code><nowiki>[[Trademark|™]]</nowiki></code> at that point. The displayed page will therefore contain the wikilink "[[Trademark|™]]".

For example, type <code>The trademark symbol is {{tl|TM}}</code> and you will see "The trademark symbol is {{TM}}" when previewing the page or after saving the change.

The other way to use a template is to [[Help:Substitution|substitute]] it. If you type <code>The trademark symbol is {{tls|TM}}</code> and preview or save the page, you will see "The trademark symbol is [[Trademark|™]]" just as with the transclusion example above. But if you save the page and then look again at the saved page <em>source</em>,{{efn|There is no way to preview the effect on the page source of substitution without saving the page}} you will see <code>The trademark symbol is <nowiki>[[Trademark|™]]</nowiki></code>, because the template call was replaced by the expansion of the template when you saved the page. There is no ongoing connection to the template call; no one can tell by looking at the page source that you didn't just type <code>The trademark symbol is <nowiki>[[Trademark|™]]</nowiki></code> and if the {{tl|TM}} template changes (or ceases to exist), your page will continue to display as it does now.  (In contrast, in the transclusion example above, if the {{tl|TM}} template subsequently changes, your page may then display differently).

===={{anchor|Parameters}} Examples with parameters====

An example of a template that takes parameters is the template {{tlx|about}}. Try typing {{tnull|about|how to use templates|how to use modules|Help:Lua}} in the sandbox—it will produce the following text:
{{about|how to use templates|how to use modules|Help:Lua}}
The template {{tlx|about}} uses three [[unnamed parameter]]s (also called positional parameters) in the example above, but you can call the same template with different numbers of parameters to give slightly different results, as explained in [[Template:About/doc|the template's documentation]]. For example, {{tnull|about|&#124;how to use modules|Help:Lua}}. Note the usage of an empty parameter—in this instance, the consecutive pipes mean that the first parameter specified is an empty string, which in this template will cause it to omit the initial "about" sentence. This produces: {{about||how to use modules|Help:Lua}} .

A simple example of a template that has named parameters is [[Template:Payoff matrix]], used to generate a 2-by-2 grid. For example:
{{Markup|<nowiki>{{payoff matrix | UL = 5 | UR = 7 | DL = 2 | DR = 9 | Name = Example usage }}</nowiki>|{{payoff matrix | UL = 5 | UR = 7 | DL = 2 | DR = 9 | Name = Example usage }}}}

See the [[Template:Payoff matrix|template page]] for more possibilities. Notice that the template is called here without specifying all its possible parameters, so undefined parameters are given default values.

===Hints and workarounds===
The following points may be worth noting when using templates:
* Templates are not the only method of [[Help:Transclusion|transclusion]], in some cases, other methods such as [[WP:SELECTIVETRANSCLUSION|selective transclusion]] are more appropriate.
* {{anchor|Equals}}An unnamed parameter value cannot contain an ordinary equals sign, as this would be interpreted in the parameter specification as a named parameter (with the equals sign separating the name from its value).{{efn|This does not apply if the equals sign comes within another template call or other item which the parser handles separately.}} To specify an unnamed parameter including an equals sign (for example in a [[URL]] with [[name–value pair]]s), replace the equals sign with a call to template {{tl|{{=}}}}, which expands to an equals sign that will not be interpreted. Another method is to replace the unnamed parameter specifications with named parameters specifications—unnamed parameters are equivalent to named parameters where the first unnamed parameter is named "1" and so on. To call template {{tl|done}} with <code>a=b</code> as the literal value for the first parameter, type either <code><nowiki>{{done|a{{=}}b}}</nowiki></code> or <code><nowiki>{{done|1=a=b}}</nowiki></code>.
* Similarly, it is not possible to use an ordinary pipe character <code>&#124;</code> in a template parameter specification, as it would be interpreted as separating one parameter specification from another.{{efn|Again, this does not apply if it comes within another separately parsed item, such as a piped wikilink.}} This problem can similarly be solved by using the [[mw:Help:Magic words#Other|magic word]] {{tlx|!}} in place of the pipe, or—if the pipe is not intended to be parsed at a higher level—using the [[List of XML and HTML character entity references|HTML entity]] <code>&amp;#124;</code>. Alternatively, for embedding [[Help:Table|wikitables]] in templates, you may use {{tlx|Wikitable}} to avoid excessive {{tlx|!}}.
* Remember that whitespace characters (spaces, tabs, carriage returns, and line feeds) are not automatically stripped from the start and end of unnamed parameters, unlike with named parameters. Including such characters (or any other non-visible characters in any parameters) may in some cases affect the template's behavior in unexpected ways. (Template designers can use {{tl|Trim}} to remove unwanted whitespace in unnamed parameters).
* In documentation and discussions, it is customary to put the name of a template in double braces to emphasize the reference to a template (for example, use {{tl|Trim}} as the name of Template:Trim).  If you just type <code><nowiki>{{Trim}}</nowiki></code> in the source page, that will of course call the template, so to make it easy to display the name with the braces, and also make the name a link to the template for the reader's convenience, there is the {{tl|tl}} template (the "'''t'''emplate '''l'''ink" template).  For example, {{tnull|tl|Example}} produces {{tl|Example}}. There are various other [[Template:Template-linking templates|template-linking templates]] available with other functions.
* When an editor changes a template (by editing the template or one of its subtemplates), the change will ultimately be reflected any time any page that transcludes that template is subsequently displayed. However, the change may not become visible on all pages immediately; a previously cached version of a page, based on the previous version of the template, may continue to be displayed for some time. Use the [[WP:Purge|purge]] function to force a page to be displayed using the latest versions of templates—including on the template page itself, if it contains usage examples.
* When viewing old versions of pages, remember that templates will be transcluded as they are now, not necessarily as they were when the old page version was active.
* To list all pages that transclude a template, use the [[Help:What links here|What links here]] link on the page when you display the template page. Note that this will {{em|not}} include pages where the template has been substituted, as there is no record of substitutions.
* To get a list of templates transcluded on a page, bring up the edit page and find the list below the edit window. This list also includes the subtemplates used by the templates that are directly transcluded. To get such a list for a page section, an old version of the page,{{efn|For an old version, the subtemplate tree will be constructed according to the templates' current state.}} or your newly edited version prior to saving, click {{Button|Show preview}} on the appropriate edit page.
* There are limits to the number and complexity of the templates that an article may have. See the "[[#Expand limits|§ Expand limits]]" section for help with this.
* If you want the template to leave a [[WP:~|time stamp or signature]], you can write <code>&lt;noinclude>&lt;nowiki>&lt;/noinclude><nowiki>~~~~~</nowiki>&lt;noinclude>&lt;/nowiki>&lt;/noinclude></code>, but this works only when you <em>substitute</em> the template. If you <em>transclude</em> it, you'll just get <nowiki>~~~~~</nowiki>.
* To improve readability, programmers usually like to split code with newlines and indent it. Unfortunately, [[MediaWiki]] does not allow this technique; in many instances, these purpose-built newlines are treated by MediaWiki as content. One possible workaround is to add <code>&lt;!--</code> before each newline character and <code>--></code> after it, which produces an [[HTML comment]].

==Writing templates==
<!--[[Help:Subtemplates]] redirects here.-->

===Process===
Templates are just Wikipedia pages.  They are created, deleted, and edited in much the same way as any other page.

====Creating====

To create a template, choose an appropriate name, navigate to that page, then click the "Edit" tab or create a new page as needed.

Templates are normally placed in the [[WP:Template namespace|template namespace]], though you can place a template intended for your own personal use or for experimentation in your own [[WP:User pages|user space]].

Before creating a template, do a quick search for existing templates (such as by exploring [[:Category:Wikipedia templates]]) to see if there is already a template that does what you want or a similar template whose code can be copied and modified (or left in place and expanded). Look for generic templates on which the new template can be based; for example, you can create a [[WP:Navbox|navbox]] template easily by creating a brief template that calls the generic [[Template:Navbox]].

There is no hard rule about what name to choose for a template—make it short but reasonably descriptive. If similar templates exist, try to follow a consistent naming pattern. You can rename a template without breaking existing [[Help:Transclusion|transclusions]] (what is called ''breakage'') by leaving a [[WP:Redirect|redirect]] to the new template name.

====Modifying====
Edit a template the same way as any other page: navigate to the template and click the "Edit" tab.

Be extremely careful when editing existing templates—changes made can affect a large number of pages, often in ways you might not expect. For this reason many high-use templates are [[WP:Protection policy|protected]] against editing except by [[WP:Administrators|administrators]] and [[WP:Template editor|template editors]]; other editors can propose changes on the talk page. Some templates offer a [[WP:Template sandbox and test cases|sandbox and test cases]] for experimentation.

====Deleting====
Unused or inappropriate templates should be deleted.  Templates that can be easily merged into another should be merged.

To propose the deletion or merger of a template, go to [[WP:Templates for discussion|Templates for discussion]] (TfD).

===Coding a template===
Anything that can be included on a normal page or article can be included on a template, including other templates (called ''{{dfn|subtemplates}}''). Templates often make use of programming features—parameters, parser functions, and other [[Help:Magic words|magic words]]—which allow the transcluded content to vary depending on context. There are also special tags to control which information is transcluded and which is not.

===Handling parameters===
In template code, the value of a parameter is represented by items enclosed between {{em|triple}} braces, which is a ''parameter reference''.
* The code <code><nowiki>{{{xxx}}}</nowiki></code> expands to the value of the parameter named "xxx".
* The codes <code><nowiki>{{{1}}}</nowiki></code>, <code><nowiki>{{{2}}}</nowiki></code>, and so on are expanded to the first, second, and so on unnamed parameters.  (Note that an unnamed parameter can alternatively be specified in a template call as an equivalent named parameter named "1", "2", etc.).

If a parameter is not specified in the template call, then the parameter reference is not replaced with anything -- it is expanded literally; this means that if the template call does not specify the parameter "xxx", the wikitext <code><nowiki>{{{xxx}}}</nowiki></code> inside the template expands to literally ''<nowiki>{{{xxx}}}</nowiki>'' (not the null string you may have expected). You can get a more useful behavior by specifying a default value in the parameter reference. Do this with the ''pipe syntax'': <code><nowiki>{{{xxx|dflt}}}</nowiki></code> specifies the default value <code>dflt</code> for the named parameter "xxx", and <code><nowiki>{{{1|dflt}}}</nowiki></code> specifies the default value <code>dflt</code> for the first unnamed parameter. Most often, one specifies a null default value, such as <code><nowiki>{{{1|}}}</nowiki></code> or <code><nowiki>{{{xxx|}}}</nowiki></code>.

You can use default parameter values to effect a parameter alias:
For example, if parameters "text" and "message" are names for the same parameter, which can also be specified as the only unnamed parameter, then code <code><nowiki>{{{message|{{{text|{{{1|}}}}}}}}}</nowiki></code>. If the template call specifies more than one of those parameters, then "message" will have priority, followed by "text", and finally by the first unnamed parameter. So if a template call specifies parameters <code>|message=A|text=B</code>, the above wikitext expands to <code>A</code>.

Because of the multiple meanings of double-brace and triple-brace syntax in wikitext, expressions can sometimes be ambiguous. It may be helpful or necessary to include spaces to resolve such ambiguity. For example, <code><nowiki>{{ {{{xxx}}} }}</nowiki></code> or <code><nowiki>{{{ {{xxx}} }}}</nowiki></code>, rather than typing five consecutive braces, may be more human-readable. But watch out for unwanted whitespace appearing in the template expansion.

Parameter references do not get expanded when they are wrapped in {{xtag|nowiki}} tags.

====Example====
The parameter usage example above refers to the {{tl|payoff matrix}} template.  Here is the code in the template that implements those parameters:

<syntaxhighlight lang="wikitext">
{| id="Payoff matrix" style="background:white; float: {{{Float|right}}}; clear:right; text-align:center;" align={{{Float|right}}} cellspacing=0 cellpadding=8 width={{{Width|225}}}
|-
|style="width:33%;                                                     "| 
|style="width:33%;                      border-bottom: solid black 1px;"| {{{2L|Left}}}
|style="width:33%;                      border-bottom: solid black 1px;"| {{{2R|Right}}}
|-
|style="border-right:  solid black 1px; text-align: right;             "| {{{1U|Up}}}
|style="border-right:  solid black 1px; border-bottom: solid black 1px; background:{{{ULc|white}}}; font-size:120%; "| {{{UL|0, 0}}}
|style="border-right:  solid black 1px; border-bottom: solid black 1px; background:{{{URc|white}}}; font-size:120%; "| {{{UR|0, 0}}}
|-
|style="border-right:  solid black 1px; text-align: right;             "| {{{1D|Down}}}
|style="border-right:  solid black 1px; border-bottom: solid black 1px; background:{{{DLc|white}}}; font-size:120%; "| {{{DL|0, 0}}}
|style="border-right:  solid black 1px; border-bottom: solid black 1px; background:{{{DRc|white}}}; font-size:120%; "| {{{DR|0, 0}}}
|-
|style="font-size: 90%;" colspan=3 |''{{{Name|{{PAGENAME}}}}}''
|}</syntaxhighlight>

The entity <syntaxhighlight lang="wikitext" inline>{{{2L|Left}}}</syntaxhighlight> instructs the template to use the value of the named parameter <code>2L</code> or the text <code>Left</code> if the parameter is not specified in the call.

====Special case: parameters within an XML-style opening tag====
Parameter references aren't expanded inside [[XML]]-style opening tags. Thus, the following will not work within a template:
*{{!mxt|<nowiki><ref name={{{param}}}> Smith, Adam (1776)...</ref></nowiki>}}
because the parameter is not expanded. Instead, you can use the <code><nowiki>{{#tag:}}</nowiki></code> [[WP:PF|parser function]], which is—for example—used in {{tlx|sfn}} to generate the {{tag|ref}} element; see also {{section link|Help:Magic words|Formatting}}. Therefore, the following example will work:
*{{mxt|<nowiki>{{#tag:ref | Smith, Adam (1776)... | name={{{param}}} }}</nowiki>}}

====Caution: overextending URLs====
If a parameter's value is (or ends with) a [[URL]], check whether it is displayed in Wikipedia with the link overextending by one or more characters after the URL so that clicking the link causes an error or failure. 
This could happen because the source code does not have a space after the URL or it contains or generates a space that is discarded in the processing.  Ensure that in the template expansion a soft space ({{em|not}} a [[Non-breaking space|hard or non-breaking space]]) follows the URL, regardless of whether you or a user supplied the URL or whether it was generated by automated processing. The {{tlx|spaces}} template may be useful.

===System variables and conditional logic===

Template code often makes use of the variables and parser functions described at [[Help:Magic words]] to make the template's behavior depend on the environment in which it is included (such as the current time or namespace). Parser functions can be used for some arithmetic calculations and string manipulations on variables and parameter values, but certain standard programming features such as loops and variable assignment are not available. Full string manipulation not available; some templates providing such function have been created, but they are inefficient and imperfect.

Some of the most frequently used variables and functions are listed below. For more, see [[Help:Magic words]] and the fuller documentation at the MediaWiki pages [[mw:Help:Magic words]] and [[mw:Help:Extension:ParserFunctions]].

{| class="wikitable" style="text-align:center;" width="100%"
|+Examples of core parser functions
! width="30%" | Description !! width="40%" | wiki source !! width="30%" | Displayed text
|-
| Uppercasing text
| <syntaxhighlight lang="wikitext" inline>{{uc: Heavens to BETSY! }}</syntaxhighlight>
| {{uc: Heavens to BETSY! }}
|-
| Lowercasing text
| <syntaxhighlight lang="wikitext" inline>{{lc: Heavens to BETSY! }}</syntaxhighlight>
| {{lc: Heavens to BETSY! }}
|-
| Getting a namespace name
| <syntaxhighlight lang="wikitext" inline>{{NS: 1 }}</syntaxhighlight>
| {{NS: 1 }}
|-
| Getting a Wikipedia URL
| <syntaxhighlight lang="wikitext" inline>{{fullurl: pagename }}</syntaxhighlight>
| {{fullurl: pagename }}
|}

The ParserFunctions extension provides more programming-oriented parser functions:

{| class="wikitable" style="text-align:center;" width="100%"
|+Examples of extension parser functions
! width="30%" | Description !! width="40%" | Wiki source !! width="30%" | Displayed text
|-
| rowspan="2" | Testing for equality between two strings (or parameters)
| <syntaxhighlight lang="wikitext" inline>{{#ifeq: yes | yes | Hooray...! | Darn...! }}</syntaxhighlight>
| {{#ifeq: yes | yes | Hooray...! | Darn...! }}
|-
| <syntaxhighlight lang="wikitext" inline>{{#ifeq: yes | no | Hooray...! | Darn...! }}</syntaxhighlight>
| {{#ifeq: yes | no | Hooray...! | Darn...! }}
|-
| Testing whether a string (or parameter) contains anything (other than whitespace)
| <syntaxhighlight lang="wikitext" inline>{{#if: {{{param|}}} | Hooray...! | Darn...! }}</syntaxhighlight>
| {{#if: {{{param|}}} | Hooray...! | Darn...! }}
|-
| [[Help:Calculation|Making a calculation (mathematics)]]<br />[area of circle of radius&nbsp;4, to 3&nbsp;decimal places]
| <syntaxhighlight lang="wikitext" inline>{{#expr: ( pi * 4 ^ 2 ) round 3 }}</syntaxhighlight>
| {{#expr: ( pi * 4 ^ 2 ) round 3 }}
|-
| [[Help:Calculation|Testing the result of a calculation]]<br />[is 1230 even or odd?]
| <syntaxhighlight lang="wikitext" inline>{{#ifexpr: 1.23E+3 mod 2 | Odd | Even }}</syntaxhighlight>
| {{#ifexpr: 1.23E+3 mod 2 | Odd | Even }}
|}

{| class="wikitable" style="text-align:center;" width="100%"
|+Examples of system variables
! width="30%" | Description !! width="40%" | Wiki source !! width="30%" | Displayed text (for this help page)
|-
| rowspan="2" | Page names
| <syntaxhighlight lang="wikitext" inline>{{PAGENAME}}</syntaxhighlight>
| {{PAGENAME}}
|-
| <syntaxhighlight lang="wikitext" inline>{{FULLPAGENAME}}</syntaxhighlight>
| {{FULLPAGENAME}}
|-
| Name of the current namespace
| <syntaxhighlight lang="wikitext" inline>{{NAMESPACE}}</syntaxhighlight>
| {{NAMESPACE}}
|-
| Number of registered users
| <syntaxhighlight lang="wikitext" inline>{{NUMBEROFUSERS}}</syntaxhighlight>
| {{NUMBEROFUSERS}}
|-
| Number of pages in a given category
| <syntaxhighlight lang="wikitext" inline>{{PAGESINCATEGORY:"Weird Al" Yankovic albums}}</syntaxhighlight>
| {{PAGESINCATEGORY:"Weird Al" Yankovic albums}}
|-
| Current software version
| <syntaxhighlight lang="wikitext" inline>{{CURRENTVERSION}}</syntaxhighlight>
| {{CURRENTVERSION}}
|-
| Timestamp of last revision
| <syntaxhighlight lang="wikitext" inline>{{REVISIONTIMESTAMP}}</syntaxhighlight>
| {{REVISIONTIMESTAMP}}
|}

The <syntaxhighlight lang="wikitext" inline>{{PAGENAME}}</syntaxhighlight> and <syntaxhighlight lang="wikitext" inline>{{NAMESPACE}}</syntaxhighlight>variables are particularly useful, and frequently used, to change template behavior based on the context in which they are included. Templates that contain category links often do this.  For example, a cleanup template contains a category link to categorize the calling page as one which needs cleanup, so the template is likely to condition that category link on the <syntaxhighlight lang="wikitext" inline>{{NAMESPACE}}</syntaxhighlight> variable so that talk pages, user pages, and any other pages that might call the template incidentally do not get categorized as pages needing cleanup.

===Nesting templates===
{{shortcut|WP:NEST}}
A template may call another template—this is called ''{{dfn|nesting}}'' and the called template is called, in this context, a ''{{dfn|subtemplate}}''.  When WikiMedia expands the template, it expands subtemplates as the calls to them appear, so that the final product is essentially the result of expanding templates from the most deeply nested out.

While fairly straightforward in application, it involves some noteworthy quirks and tricks.

To pass a parameter value from a template call to to a subtemplate, use a parameter reference in the template call to the subtemplate.
;Example{{colon}}
:Template:A contains <code><nowiki>"the quick brown {{B|{{{3}}} }} jumps over..."</nowiki></code>.  Template:B (a subtemplate) contains <code><nowiki>'''{{{1}}}'''</nowiki></code>.   Page X calls A with <code><nowiki>{{A|apple|pear|fox}}</nowiki></code> This expands to <code><nowiki>"the quick brown '''fox''' jumps over..."</nowiki></code>.  The third unnamed parameter passed to Template:A gets passes as the first unnamed parameter to subtemplate B.

A template can even choose which subtemplate parameter to pass conditionally.
;Examples{{colon}}
:Template:A contains <code><nowiki>the quick brown {{B|{{{1}}}=fox}} jumps over...</nowiki></code>.  Template:B (a subtemplate) contains <code><nowiki>'''{{{jumper}}}'''</nowiki></code>.  Page X calls A with <code><nowiki>{{A|apple|pear|jumper}}</nowiki></code>.  This expands to <code><nowiki>"the quick brown '''fox''' jumps over..."</nowiki></code>.  The third unnamed parameter passed to Template:A is passed as the name of the parameter passed to subtemplate B with the value "fox".

{{shortcut|WP:TEMPLATE LOOP}}
A template can call itself, but the recursion will stop after one nesting to prevent an infinite loop. Attempting to go beyond one nesting produces an error message and causes the page to be marked by MediaWiki as having a template loop.

When a subtemplate contains unmatched braces—as in <code><nowiki>{{lb}}}</nowiki></code>—the unmatched braces are treated as text during processing—they do not affect the parsing of braces in the calling template. But where the template is substituted, the unmatched braces will be parsed as braces when the page is subsequently displayed.  This has little practical use, but can occasionally introduce unexpected errors.

See [[m:Help:Advanced templates]] and [[m:Help:Recursive conversion of wikitext]] for more information.

===<span id="Noinclude, includeonly, and onlyinclude"></span>Inclusion control: noinclude, includeonly, and onlyinclude===
{{shortcut|WP:NOINCLUDE|WP:INCLUDEONLY|WP:ONLYINCLUDE}}
By default, when a page calls a template, MediaWiki includes the expansion of the entire template in the calling page. However, it is possible to modify that behavior, using tags that specify which parts of the template code are to be included. This makes it possible for the template to contain information only for display when the template page itself is displayed, such as the template's [[Wikipedia:Template documentation|documentation]], or [[WP:CAT#T|categories]]. It is also possible to have parts of the template be included in calling pages, but {{em|not}} be displayed when the template page itself is displayed and not be processed when the template page itself is saved (e.g., categories to be applied to calling pages which do not apply to the template). The tags are as follows:
* {{xtag|noinclude|p}}&nbsp;– The text between the tags is {{em|not}} included when the template is called, but {{em|is}} processed when the template itself is displayed or saved; a common use is for [[WP:Template documentation|documentation in templates]].
* {{xtag|onlyinclude|p}}&nbsp;– Nothing on the page {{em|except}} what appears between the tags is included when the template is called.
* {{xtag|includeonly|p}}&nbsp;– The text between the tags {{em|is}} included when the template is called, but is {{em|not}} processed when the template is displayed or saved.

{| class="wikitable" style="text-align: center;"
! Wikitext
! What is rendered {{em|here}} (template page)
! What is included {{em|there}} (calling page)
|-
|<code><nowiki><noinclude> text1 </noinclude> text2</nowiki></code>
|<code>text1 text2</code>
|<code>text2</code>
|-
|<code><nowiki>&lt;onlyinclude&gt; text1 &lt;/onlyinclude&gt; text2</nowiki></code>
|<code>text1 text2</code>
|<code>text1</code>
|-
|<code><nowiki><includeonly> text1 </includeonly> text2</nowiki></code>
|<code>text2</code>
|<code>text1 text2</code>
|-
|<code><nowiki>&lt;onlyinclude&gt;<includeonly> text1 </includeonly>&lt;/onlyinclude&gt; text2</nowiki></code>
|<code>text2</code>
|<code>text1</code>
|}

Perhaps the most common issue with the use of these blocks is unwanted spaces or lines. It is important to remember that the effect of these tags begins immediately before the first angle bracket, not on the previous line or at the previous visible character; similarly the effect ends immediately after the last angle bracket, not on the next line or with the next visible character. For example:

:{{mxt|<nowiki>}}<includeonly></nowiki>}}
:{{!mxt|<nowiki>}}</nowiki><br /><nowiki></includeonly></nowiki>}}

These tags can be nested inside each other, though (for a given page) this is really meaningful only for the {{tag|onlyinclude|o}} tag; nesting {{tag|includeonly|o}} and {{tag|noinclude|o}} tags is fairly pointless. Be careful to properly nest the tags, however. Constructions like {{tag|onlyinclude|o}}abc{{tag|includeonly|o}}def{{tag|onlyinclude|c}}ghi{{tag|includeonly|c}} will {{em|not}} work as expected. Use the "first opened, last closed" rule that is standard for HTML/XML.

===Problems and workarounds===
* The following techniques are helpful in debugging a template:
** Use [[Special:ExpandTemplates]] to see the full recursive expansion of one or more templates.
** Use <code>subst:</code> to substitute a template (rather than transclude it), which can show more clearly what is happening when the template is transcluded; see [[Help:Substitution]].
** Use <code>msgnw:</code> (short for "'''m'''e'''s'''sa'''g'''e, '''n'''o'''w'''iki") to more-or-less transclude the source of the template rather than its expansion. It is not perfect: lists are rendered, comments are removed, and single newlines are replaced with spaces (which is particularly confounding when transcluding wikitext tables).
* If the first character of a template expansion is one of four wiki markup characters—<code>:</code>, <code>;</code>, <code>*</code>, <code>#</code>{{efn|These are defined in the [https://web.archive.org/web/20180625163321/https://doc.wikimedia.org/mediawiki-core/master/php/classParser.html#ad463888e40c078ac9bcfcaf1231e39d7 <code>doBlockLevels</code> function of Parser.php].}}, it is processed during display as though it were at the beginning of a line, even if the template call is not. This allows you to create various kinds of lists with templates where the template call may not be in the correct place for a list. To avoid this, either use {{xtag|nowiki|s}} before the markup or use the [[HTML]] entities <code>&amp;#58;</code>, <code>&amp;#59;</code>, <code>&amp;#42;</code>, and <code>&amp;#35;</code> respectively. In some cases, the HTML entities will work when the {{tag|nowiki|s}} does not. The problem often occurs when a parameter value in a template call starts with one of the four characters. See also {{tl|Encodefirst}}. 
* For issues with template substitution, such as how to control whether subtemplates are substituted as well when the parent template is substituted, see [[Help:Substitution]].
* You can use the template {{tlx|Trim}} to strip any initial or final whitespace from unnamed parameter values if this would cause problems; <em>named</em> parameter values are automatically stripped in this way.
* To protect server resources and avoid infinite loops, the parser imposes certain limits on the depth of transclusion nesting and on the page size with expanded templates. This may cause a page to break if it uses very complex templates, particularly if there are multiple such templates on the same page. For more information, see [[WP:Template limits]]. You can check a page's overall load on the server by examining the generated HTML for a page and looking for the <code>NewPP limit report</code> comments.
* Do not use <code>=</code> wikimarkup to create a section header in a template which is intended for use in article space; this will create an edit link on a page that transcludes the template that will confusingly open the <em>template</em> for editing.
* You may avoid section edit links to the template by including <code><nowiki><includeonly>__NOEDITSECTION__</includeonly></nowiki></code>.

===Documentation===
{{Details|WP:Template documentation}}
Documentation for users, together with the template's categories, normally goes after the template code, inside {{tag|noinclude}} tags. It is normally necessary to put the opening {{tag|noinclude|o}} tag immediately after the end of the code, with no intervening spaces or newlines, to avoid transcluding unwanted whitespace.

In the case of complex templates, the documentation is often kept on a separate [[WP:Subpages|subpage]] of the template page (named "Template:{{var|XXX}}/doc"). This applies especially to many [[WP:Protection policy|protected]] templates, so that non-administrators can edit the documentation. To do this, place a call to the {{tlx|Documentation}} template after the main template code and within {{tag|noinclude}} tags. If the "/doc" subpage does not exist, a link appears when you display the template that you can use to create the subpage.

The documentation subpage, rather than the template itself, is normally what is placed in categories to represent a template.

===Categorization===
{{See also|WP:Categorization#Template categorization}}
====Categorize pages by template inclusion====
Some templates generate category declarations in their expansion, since the template is intended to place calling pages in particular categories. This is often done with maintenance categories.  Placing articles into ordinary content categories in this way is discouraged. When doing this, you may have to use {{tag|includeonly}} tags to keep the template itself out of the category. While developing, testing, sandboxing, or demonstrating a template intended to apply a category, either temporarily replace each category with a test category (starting with [[:Category:X1|X1]], [[:Category:X2|X2]], or [[:Category:X3|X3]]) or suppress categorization (see [[WP:CATSUP|category suppression in templates]]).

====Categorize templates====
Categorizing your template and documenting its proper usage will make it easier for other editors to find and use.

Category declarations for a template itself should be placed on the template's documentation subpage (or inside {{tag|noinclude}} [[html tag|tags]] if there is no documentation subpage) to avoid placing calling pages in the category.

===Aliases===

A [[WP:Redirect|redirect]] of a template functions as an alias. For example, [[Template:Tsh]] redirects to [[Template:Template shortcut]], so you can code {{tlx|tsh|{{var|foo}}}} instead of {{tlx|Template shortcut|{{var|foo}}}}.

It is good to prepare template aliases for variations in whitespace and capitalization. For example, there is a template called {{tlx|See Wiktionary}}. The "W" is capitalized, since the word "Wiktionary" is so, but a redirect {{tlx|See wiktionary}} (with lowercase "w") exists because editors may misremember it as the latter.

==Template limits==

{{Anchor|Expand limits}}
{{details|WP:Template limits}}
'''"Post-expand include size" limit.''' When templates are rendered or expanded to HTML for viewing in your browser, they use memory. This is called the "post-expand include size" and has a limit of 2,048,000&nbsp;bytes. This size is included as an invisible comment in the HTML output—use your browser's view source feature to show the raw HTML and search for "newpp". The report will look like:
<syntaxhighlight lang="html4strict">
<!--
NewPP limit report
Preprocessor node count: 2382/1000000
Post-expand include size: 63476/2048000 bytes
Template argument size: 9517/2048000 bytes
Expensive parser function count: 2/500
-->
</syntaxhighlight>

The example shows that template expansion is using 63,476&nbsp;bytes out of 2,048,000&nbsp;bytes of available memory.

'''Display problem.''' If too many templates are included on a page, the post-expand include size may exceed the limit. When this happens, templates after the limit will no longer expand and will instead display as a wikilink (for example, {{red|1=<u>Template:{{var|Template name}}</u>}}). Common causes are the inclusion of too many citation templates and/or flag templates. To resolve this problem [[WP:Substitution|substitute]] templates, remove templates, or [[WP:Splitting|split]] the page.

'''Non-rendered tranclusions still count towards limit.''' For example, a page which contains only <code><nowiki>{{#if:{{:Main Page}}}}</nowiki></code> would still have a post-expand include size even though it would have no output at all.

The same applies to [[WP:Lua|Scribunto modules]]. For example, <code><nowiki>{{#invoke:Test|main}}</nowiki></code> would still increase post-expand include size even if Module:Test were simply:
<syntaxhighlight lang="lua">
mw.getCurrentFrame():preprocess "{{msgnw::Main Page}}" -- remove this line and post-expand include size becomes zero
return {main = function() end} -- p.main() has no return value

</syntaxhighlight>

==Lua programming language==
{{main article|WP:Lua}}
The [[Lua (programming language)|Lua programming language]] is available for use through the [[mw:Extension:Scribunto|Scribunto]] MediaWiki extension. You can embed Lua code into templates by employing the <code><nowiki>{{#invoke:}}</nowiki></code> functionality of the Scribunto MediaWiki extension. The Lua [[source code]] is stored in pages called modules, and templates invoke these individual modules. For example, you can invoke [[Module:Example]] with the code <code><nowiki>{{#invoke:Example|hello}}</nowiki></code> to print the text "{{#invoke:Example|hello}}".

==Terminology==
A ''template'' is a Wikipedia page whose purpose is to be transcluded or substituted in another page.  Templates are usually in the [[WP:Template namespace|template namespace]], but don't have to be.  Pages can transclude or substitute any page, not just templates; what makes a page a template is the page's purpose.

''Calling'' a template means transcluding or substituting one.  A page calls a template.  The wikitext on a page that causes the page to call a template is called a ''template call''.  For example, <code><nowiki>{{sic|constellation prize}}</nowiki></code>.  Template calls are always delimited by double braces (<code><nowiki>{{}}</nowiki></code>), but not everything enclosed in double braces is a template call.

''Expansion'' is the process of generating page source from a template, when a page calls it, by applying parameters and other features.  MediaWiki expands a template as it renders a page.  One can also say that the template itself expands, so both "MediaWiki expands <code><nowiki>{{{1}}}</nowiki></code> as the value of the first positional parameter" and <code><nowiki>{{{1}}}</nowiki></code> expands as the value of the first positional parameter" work.

''Expansion'' of a page is also the page source that results from template expansion.  For example, the expansion of the <nowiki>{{sic}}</nowiki> template when called by <code><nowiki>{{sic|constellation prize}}</nowiki></code> is <code><nowiki>constellation prize&amp#32;&amp#91;''[[sic]]''&amp#93;</nowiki></code>.

The displayed page content generated by a template call (which is the rendering of the expansion of the called template) is the ''template result''.  The template result generated by the template call <code><nowiki>{{sic|constellation prize}}</nowiki></code> is "{{sic|constellation prize}}".

The ''name'' of a template is the name of the wikipedia page (which is also the title of the page).  The namespace is normally left off if it is "Template" (which it almost always is).  Furthermore, it is customary to write the name with double braces, like a template call, to emphasize that it names a template.  For example, "Use the <nowiki>{{tm}}</nowiki> template to generate the trademark symbol."  But don't go so far as to put a template name in the code font, to prevent confusion with an actual template call.  <nowiki>{{tm}}</nowiki> is a template name, but {{tnull|tm}} is a template call.

Some template calls perform a tagging function; such a template call is often called a ''tag'', as in, "If you are in the middle of a major edit, place an <nowiki>{{in use}}</nowiki> tag at the top of the page."  This is one of many ways the term "tag" is used in Wikipedia.

A ''parameter'' is a particular piece of information that can be specified in a template call to affect the expansion of the template.  A parameter has a ''value''.  A named parameter has a ''name''; an unnamed parameter has a ''position''.  A template call ''specifies'' a parameter and the code that does that is a ''parameter specification''.  A template ''defines'' a parameter (not explicitly -- just by the template being designed to use it).  The code in a template that expands to the parameter value (e.g. <code><nowiki>{{{myparam}}}</nowiki></code>) is a ''parameter reference''.

===Common variations===
It is common to use "template" to refer not only to a template, but to a template call, a template result, and a template name.  For example: 

 I am going to put more detail in the <nowiki>{{short description}}</nowiki> template on this page.

 Upon seeing an <nowiki>{{under construction}}</nowiki> template, the reader may decide  to come back later.

 There are many kinds of infoboxes.  The templates all start with "Infobox".

These usages can be confusing, especially to someone new to templates, so avoid them.

==Template search==
{{main|WP:Template index}}
As an alternative to using this index linked above, you can search the Template namespace using the [[Special:Search]] box below:

<inputbox>
type=fulltext
width=25
namespaces=Template**
searchbuttonlabel=Search Template namespace
break=no
placeholder=e.g. Citation
</inputbox>

==See also==
=== Help pages ===
{{div col|colwidth=20em}}
* Introductions:
** [[Help:A quick guide to templates]]
** [[Help:Substitution]]<!-- maybe portable, test it -->
** [[Help:Transclusion]]
* [[Help:Magic words]]
* [[Help:Manipulating strings]]
* [[Help:Metatemplating]]
* [[WP:Advanced template coding]]
* [[WP:Template documentation]]
* [[WP:Template index]]
* [[WP:Template limits]]
* [[WP:Template namespace]]
* [[WP:Template sandbox and test cases]]
* [[WP:TemplateData]]—standardized template description used by [[Wikipedia:VisualEditor|VisualEditor]]
* [[WP:WikiProject Templates]]
* [[m:Help:Advanced templates]]<!-- m: not directly portable due to examples -->
* [[m:Help:Calculation]]
* [[m:Help:Parameter default]]
* [[m:Help:Template]]
* [[m:User:Happy-melon/Templates]]
* [[mw:Help:ExpandTemplates]]
* [[mw:Help:Extension:ParserFunctions]]
* [[mw:Help:Magic words]]
* [[mw:Help:Templates]]
{{div col end}}
=== Mediawiki manual pages ===
* [[mw:Manual:$wgEnableScaryTranscluding]]
* [[mw:Manual:$wgNonincludableNamespaces]]
* [[mw:Manual:Parser.php]]
* [[mw:Manual:Parser functions]]

=== Special pages ===
* [[WP:Database reports/Templates transcluded on the most pages]]—not counting transclusions through redirects
* [{{fullurl:Special:Allpages|namespace=10}} Special:Allpages (namespace 10)]—all pages in the Template namespace
* [[Special:ExpandTemplates]]
* [[Special:Unusedtemplates]]—templates which are not transcluded (but may be used for substitution)

=== Other backend pages ===
* [[Help:User style]]—CSS and JS pages
* [[Help:System message]]—MediaWiki namespace pages
* [[m:Project portals]]—Multilingual portal pages
* [[WP:Book sources]]—used by [[Special:Booksources]]
* [[m:Spam blacklist]]
* [[Template:Example]]

==Notes==
{{Notelist}}

{{Wikipedia template messages}}
{{Wikipedia help pages}}
{{Wikipedia technical help}}

[[Category:Wikipedia how-to]]
[[Category:Wikipedia template help]]