JSFiddle - React, Tailwind, and code Playground

HTML

<textarea name="" id="text" cols="30" rows="10">hello
world
</textarea>
<button id="pew">pew</button>

JavaScript

pew.onclick = () => {
  var text = document.getElementById('text').value;
  text = text.replace('w', 'foobar\nw')
  document.getElementById('text').value = text;
}