Detect which form input has focus using JavaScript or jQuery

I am not sure if this is the most efficient way, but you could try:

var selectedInput = null;
$(function() {
    $('input, textarea, select').focus(function() {
        selectedInput = this;
    }).blur(function(){
        selectedInput = null;
    });
});

Try

window.getSelection().getRangeAt(0).startContainer