JSFiddle - React, Tailwind, and code Playground

HTML

<div id="editor" class="editor" contentEditable="true">Hallo</div>

<input type="button" value="Bold" onclick="appendB()"/>

JavaScript

function appendB(){
    var bold = document.createElement("b");  
    bold.innerHTML = "&nbsp;";
    //create it            
    document.getElementById('editor').appendChild(bold);    //append it
    setFocus();
}

function setFocus() {
    var el = document.getElementById("editor");
    var range = document.createRange();
    var sel = window.getSelection();
    range.setStartAfter(el.lastChild);
    range.collapse(true);
    sel.removeAllRanges();
    sel.addRange(range);
    el.focus();
}