condition.variable.isset ViewHelper <vhs:condition.variable.isset> 

Variable: Isset 

Renders the then child if the variable name given in the name argument exists in the template. The value can be zero, NULL or an empty string - but the ViewHelper will still return TRUE if the variable exists.

Combines well with dynamic variable names:

<!-- if {variableContainingVariableName} is "foo" this checks existence of {foo} -->
<v:condition.variable.isset name="{variableContainingVariableName}">...</v:condition.variable.isset>
<!-- if {suffix} is "Name" this checks existence of "variableName" -->
<v:condition.variable.isset name="variable{suffix}">...</v:condition.variable.isset>
<!-- outputs value of {foo} if {bar} is defined -->
{foo -> v:condition.variable.isset(name: bar)}
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ONLY WORKS ON TYPO3v8+!

Arguments 

then 

DataType
mixed
Required
false
Description
Value to be returned if the condition if met.

else 

DataType
mixed
Required
false
Description
Value to be returned if the condition if not met.

name 

DataType
string
Required
true
Description
Name of the variable