JSFiddle - React, Tailwind, and code Playground

HTML

<textarea><a href="http://www..com"></a></textarea>

JavaScript

var txt = $('textarea').text();
var ancStart = txt.toString().indexOf('<a');
var ancEnd = txt.toString().indexOf('a>') + 2;
var sl = (ancStart * -1) + ancEnd;
var newTxt = txt.slice(sl);
newTxt += '<a href="http://www.stackoverflow.com"></a>';
$('textarea').empty().text(newTxt);