JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id="mytext" rows="10" cols="30">
Really long text goes here
</textarea>
<br/>
<button onclick="doIt();">Click Me</button>

JavaScript

var doIt = function() {
    var elem = document.getElementById("mytext");
    var start = elem.selectionStart;
    var end = elem.selectionEnd;
    var selection = elem.selection;
    alert("selectionStart: " + start 
          + "\nselectionEnd :" + end
          + "\nselection: " + selection);
}