JSFiddle - React, Tailwind, and code Playground

HTML

<textarea></textarea>

CSS

textarea {
  margin:20px;
  width: 300px;
  height:300px;
}

JavaScript

var textArea = document.getElementsByTagName('textarea')[0];

function mouseMove(e) {
  var x = e.pageX - textArea.offsetLeft,
      y = e.pageY - textArea.offsetTop;
  
  textArea.value = x + ", " + y;
}

textArea.addEventListener('mousemove', mouseMove, false);