How can I insert text via button click at the cursor in the most recent textbox used?

I have a form with multiple text boxes and a set of buttons. Using the Javascript below, I am able to click a button and insert text into one of the named boxes.

Is it possible to insert text into the most recent/active textbox when a button is clicked?

Currently I have this, but it's using the specific ID of a text box, rather than the most recently used/active one;

   $( 'input[type=button]' ).on('click', function(){
      var cursorPos = $('#my_text_box').prop('selectionStart');
      var v = $('#my_text_box').val();
      var textBefore = v.substring(0,  cursorPos );
      var textAfter  = v.substring( cursorPos, v.length );
      $('#my_text_box').val( textBefore+ $(this).val() +textAfter );
    });

Example on JSFiddle for more clarity;

http://jsfiddle.net/bd1xf0sj/1/

The idea being that depending upon which text box you're in when the button is clicked, instead of using the ID of a certain textbox. The idea being that the buttons can be clicked and any textbox can be used.

from How can I insert text via button click at the cursor in the most recent textbox used?