get span of selected text

Is there a "dirty" way to use javascript to get the position of a selected text? I have a textbox (or a formula generated text, if that helps) and a user needs to select a text where I would need to store the start and end indices to the span. A search is not a option as there's often no unique match.

Something like https://docs.annotatorjs.org/en/v1.2.x/ or https://markjs.io/

It can be very basic though