Marking text in textarea with b and i tags

userscript.js

// ==UserScript==
// @name       Notabenoid bbcode shortkey
// @namespace  http://notabenoid.zapto.org/
// @version    0.1
// @description  enter something useful
// @match      http://notabenoid.zapto.org/*
// @copyright  2019, topin89@mail.ru
// ==/UserScript==

function changeSel(chr) // javascript
{
    // obtain the object reference for the <textarea>
    var txtarea = document.activeElement;

    var isInputText = txtarea instanceof HTMLTextAreaElement;

    if(!isInputText) {return ;}

    // obtain the index of the first selected character
    var start = txtarea.selectionStart;
    // obtain the index of the last selected character
    var finish = txtarea.selectionEnd;

    var str = txtarea.value

    var opentag = '<'+chr+'>'
    var closetag = '</'+chr+'>'
    var new_str = ''

    if(str.includes(opentag) && str.includes(closetag)){
        new_str =str.replace(opentag, '').replace(closetag, '')
    }else{
        new_str =str.substring(0,start) + opentag + str.substring(start,finish) + closetag + str.substring(finish);
    }
    txtarea.value = new_str;
    txtarea.setSelectionRange(start, finish + new_str.length - str.length);

}

function doc_keyDown(e) {
    if(e.ctrlKey && !e.altKey && !e.metaKey && !e.shiftKey){
        if(e.code == "KeyB"){
            changeSel('b');
            e.stopImmediatePropagation();
            e.preventDefault();
        }else if(e.code == "KeyI"){
            changeSel('i');
            e.stopImmediatePropagation();
            e.preventDefault();
        }
    }
}

document.addEventListener('keydown', doc_keyDown, true);