Little details about using a .JS Code Page to house the script for on-click changes to a record

My ultimate goal here is to find out what the .js codepage code commands should contain in order to write values to various field types.

 

The button to execute the commands is located in the record's form, in edit mode only. 

 

What I know:

 

How to check a checkbox:

   $("#_fid_xxx").attr("checked", true); 

 

How to write a value to a text box:

   $("#_fid_xxx").val("your value here")

 

How to write today's date to a date (and Date/time) field:

   $("#_fid_xxx").val("today");

 

What I do not know:

 

How to write a value to a text box from another text box:

 

How to write a username (or their email, whichever is less complicated) to a text box:

 

 

--I also have some webpage navigation scripting questions, but those could be their own thread - depending on how deep the rabbithole goes.