You can also create a property on the client side: procedure TMainForm.UniFormCreate(Sender: TObject); begin ... Self.WebForm.JSInterface.JSAssign('LDesignMode', [LDesignMode]); end; and in the setter method of your field: procedure TMainForm.SetDesignMode(const Value: Boolean); begin FDesignMode := Value; Self.WebForm.JSInterface.JSAssign('LDesignMode', [Value]); end; ...