How to get the current text selection with Javascript on ANY part of the website?

I'd like to get the selected text from a website, but when I use document.getSelection it does not work for selections inside textareas and to get the selection from a text area I would need to know which textarea has the selection to use the textarea functions. Is there a simple methods for getting the current selection or do I need to iterate over all elements to test if they contain selected text?

Selection of text in general document content and in user input elements are mutually exclusive.

• HTMLTextAreaElement and HTMLInputElement both have selectionStart and selectionEnd properties which are offsets of selected text in their content

• INPUT elements which don't support selection return null for the selection start/end properties.

• If selectionStart and selectionEnd have the same value there is no selection within the element, and if the values are numeric it give the position of the cursor in the element.

• If you click elsewhere on the page, INPUT and TEXTAREA elements lose focus and and any selected text in their content is deselected.

• document.activeElement holds a reference to the currently focused element. A null value indicates the user has clicked outside web page content in the browser, possibly to change tabs. The tagName property of a non-null activeElement indicates what kind of element it is in upper case.

Putting all of this together I would start with a function similar to that below - it returns null if the page if the not in focus or an INPUT element that doesn't support selection is in focus. Otherwise it returns one of two types of object: either a selection object or a custom object containing details of user input with a type property set to "UIText".

function getCurrentSelection() {
  let selection;

  const active = document.activeElement;
  if(!active) return null;

  let tagName = active.tagName;
  if( tagName == "INPUT" || tagName == "TEXTAREA") {
    const {selectionStart, selectionEnd} = active;
    selection = selectionStart === null ? null : {
     type: "UIText",
     element: active,
     tag: tagName,
     selectionStart,
     selectionEnd,
     text: active.value.substring(selectionStart, selectionEnd)
    };
   }
   else {
      selection = document.getSelection()
   }
   return selection;
}