I would recommend against implementing something like the above get_element_value for HtmlDialog — because it does not do what its name says, it does not get and return anything. Moreover the whole construct with callback is split over three methods, hardly understandable and maintainable. Actually you would rather want to have the callback together with get_element_value than separate (either as callback function parameter or as callback that can be registered on a returned promise).
Better get used to the new concept of asynchronous dialogs. Even the web is asynchronous and works well. Think of the dialog (client) sending to or requesting data from SketchUp (server).
And please put your code in code blocks (edit, </> button). That the code turns out so messy (bold, different size, characters missing) is a direct consequence from that the forum software thinks it can interprete it is markup.