JSFiddle - React, Tailwind, and code Playground

HTML

<textarea>This is a text.</textarea>

JavaScript

var textarea = document.querySelector("textarea");
textarea.addEventListener("click", function() {
	console.log(getCaretPosition(textarea));
  console.log(getSurroundingSelection(textarea));
});

function getCaretPosition(textarea) {
	return textarea.selectionStart
}
function getSurroundingSelection(textarea) {
	return [textarea.value.substring(0, textarea.selectionStart)
         ,textarea.value.substring(textarea.selectionStart, textarea.selectionEnd)
         ,textarea.value.substring(textarea.selectionEnd, textarea.value.length)]
}