How to use placeholder like {app} or {temp} in Inno Setup script?

Question

How can I access (directory) constants from Inno Setup script code ?

I have tried the following without success:

Answer 1

Use ExpandConstant function

function dbExistis() : Boolean;
  begin
    Result := FileExists(ExpandConstant('{commonappdata}') + '\LR-International\DB_LR.IB');
  end;