This template is used on many pages and changes may be widely noticed. Test changes in the template's /sandbox or /testcases subpages, or in your own user subpage. Consider discussing changes on the talk page before implementing them.
This template is used inside other templates. It takes any number of parameters and returns the first found to be defined and non-empty. If none is found, nothing is returned. By writing {{if empty|a|b|c}} instead of {{#if:a|a|{{#if:b|b|c}}}}, expression a and b do not have to be repeated and evaluated twice. The template provides a fallback order, similar to a try catch based "It is easier to ask for forgiveness than permission" (EAFP) programming style.
("Return {{{logo|}}} if it has a value; else {{{image|}}} if it has a value; else {{{picture|}}} if it has a value; else return "default.svg".")
This returns the first of the parameters logo, image and picture that is defined and non-empty, otherwise "default.svg".
Rationale
The MediaWiki parameter default function doesn't return the default value for empty parameters. That is, {{{logo|default.svg}}} does not return "default.svg" if the template was called like this: {{template|logo=}}.
The usual workaround for a single parameter is:
{{#if:{{{logo|}}} |{{{logo}}} |default.svg}}
("If {{{logo}}} has a value, return it, else return "default.svg".")
But this becomes complex when several parameters are to be checked:
("If {{{logo}}} has a value, return it; else if {{{image}}} has a value, return that; else if {{{picture}}} has a value, return that; else return "default.svg".")
In these cases, {{if empty}} produces the simpler syntax (as above):
Parameters used with {{if empty}} must be piped – i.e. include the vertical bar (pipe) symbol("|") as a trailing character – so that empty or undefined parameters aren't treated as text and returned incorrectly. Hence, for example, {{{logo|}}}, {{{image|}}} and {{{picture|}}} rather than {{{logo}}}, {{{image}}} and {{{picture}}} in the above examples.
Examples
Code
Result
{{if empty}}
Returns an empty string.
{{if empty|one}}
one
Returns the first parameter that is defined and not empty.
{{if empty|one|two}}
one
{{if empty|one|two|three|four}}
one
{{if empty|two}}
two
The first parameter is empty/undefined, so is passed over.
{{if empty|two|three|four}}
two
{{if empty|two|four}}
two
{{if empty||||||||||ten}}
ten
Unlike in previous versions, this template is no longer limited by 9 parameters.
{{if empty|}}
The only parameter is empty or undefined, so returns an empty string.
{{if empty|||||}}
Returns an empty string.
{{if empty|{{{1|}}}|{{{2|}}}|three}}
three
{{if empty|{{{1}}}|{{{2}}}|three}}
{{{1}}}
No pipe-characters following the names of the first two parameters ("1" and "2"), so the first of these returned as text ("{{{1}}}").
{{if empty|{{{logo|}}}|two}}
two
{{if empty|{{{logo}}}|two}}
{{{logo}}}
No pipe-character following the parameter name "logo", so the text "{{{logo}}}" returned.
{{if empty|p=q}}
The template identifies the parameters it receives as parameters 1 to 9, not using names such as "p", etc.
