JSFiddle - React, Tailwind, and code Playground

HTML

<div class="content" contenteditable="true">
Vivamus <span>luctus</span> urna sed urna ultricies ac tempor dui sagittis.
</div>

JavaScript

$(".content").on("mouseup", function () {
    var start = window.getSelection().baseOffset;
    var end = window.getSelection().focusOffset;
    if (start < end) {
        var start = window.getSelection().baseOffset;
        var end = window.getSelection().focusOffset;
    } else {
        var start = window.getSelection().focusOffset;
        var end = window.getSelection().baseOffset;
    }
   alert(window.getSelection());
    alert(start + ", " + end);
});