IsMissing

Return TRUE if the optional variant argument has been passed to a procedure.

Syntax
      IsMissing(argname)

Key
   argname    The name of an optional variant procedure argument.

The IsMissing() function may be used in VBA.

Examples

Function Demo1 (Optional ArgA)
If IsMissing(ArgA) Then
Msgbox "Value not supplied", vbOKCancel, "SS64 IsMissing Demo"
End If

“Sometimes only one person is missing and the whole world seems depopulated” ~ Alphonse de Lamartine

Related:

IsNull - Is expression NULL? True/False
Nz - Detect a NULL value.