TextArea Selection Change?

What you want is the global selectionchange event available on the window or document. Read about it here.

Add a unique id to your textarea:

<textarea id="unique-id"></textarea>

Add an event listener to the document:

function handleSelection() {
  const activeElement = document.activeElement

  // make sure this is your textarea
  if (activeElement && activeElement.id === 'unique-id') {
    const range = {
      start: activeElement.selectionStart,
      end: activeElement.selectionEnd
    }
    // do something with your range
  }
}

document.addEventListener('selectionchange', handleSelection)

That will run whatever code is in the handleSelection function.

You can use the onselect event

object.addEventListener("select", myScript);

https://www.w3schools.com/jsref/event_onselect.asp