Editing Module:Yesno/doc (section)

===Undefined input ('foo')===
You can specify a default value if yesno receives input other than that listed above. If you don't supply a default, the module will return <code>nil</code> for these inputs.

<syntaxhighlight lang="lua">
-- These return nil:
yesno(nil)
yesno('foo')
yesno({})
yesno(5)
yesno('')
yesno(function() return 'This is a function.' end)
yesno(nil, true)
yesno(nil, 'bar')

-- These return true:
yesno('foo', true)
yesno({}, true)
yesno(5, true)
yesno('', true)
yesno(function() return 'This is a function.' end, true)

-- These return "bar":
yesno('foo', 'bar')
yesno({}, 'bar')
yesno(5, 'bar')
yesno('', 'bar')
yesno(function() return 'This is a function.' end, 'bar')
</syntaxhighlight>

Although the empty string usually evaluates to false in wikitext, it evaluates to true in Lua. This module prefers the Lua behaviour over the wikitext behaviour. If treating the empty string as false is important for your module, you will need to convert empty strings to a value that evaluates to false before passing them to this module. In the case of arguments received from wikitext, this can be done by using [[Module:Arguments]].