identifier not found 'SizeOf'

I wanted to use SizeOf but I am getting a compilation error:

[Error] Unit1.pas (28, 7): identifier not found "SizeOf"

This comes from an empty TMS Web Application with the following FormCreate method:

procedure TForm2.WebFormCreate(Sender: TObject);
var x : integer;
  x := sizeof (x);

I noticed sizeof is used in some other TMS source files so I assumed I could used it. Using the latest stable release of TMS on Delphi 10.3 Up2. 

SizeOf() doesn't exist in pas2js compiler and it also doesn't make sense in JavaScript as it has no fixed size data types.