JSFiddle - React, Tailwind, and code Playground

HTML

<p contenteditable>Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua</p>

<div id="info">info</div>

JavaScript

document.onselectionchange = () => {
    var sel = document.getSelection()
    info.innerHTML =
       'anchorOffset: ' + sel.anchorOffset + '<br>' +
       'focusOffset: ' + sel.focusOffset + '<br>'
}