JSFiddle - React, Tailwind, and code Playground

HTML

<div id="mt">
Keens' Women Venice H2 Sandals are a great pair of versatile and stylish sandals that include a range of exciting features. Topside they are constructed using an open strapping system, which provides outstanding ventilation. Underneath, a streamlined, grippy outsole features multi-directional lugs with razor siping, providing exceptional traction control on a variety of terrains.

The EVA metatomical molded footbed on the Venice H2 provides an outstanding level of comfort on all levels and are available in a range of stylish colours that will ensure you set the outdoors ablaze this summer. Once again, Keen has provided another fantastic product that you're guaranteed to love. 
</div>

JavaScript

var getSelection = function() {
    return $try(
        function() { return window.getSelection(); },
        function() { return document.getSelection(); },
        function() { 
            var selection = document.selection && document.selection.createRange();
            if(selection.text) { return selection.text; }
            return false;
        }
    ) || false;
};

var getCaretPosition = function(selection, el) {
    var el = document.id(el) || window;
    
    var text = el.get("text");
    var offset = text.indexOf(selection);
    return offset + selection.length;
}

document.id("mt").addEvent('mouseup',function(e) {
    var selection = getSelection();
    if(selection && (selection = new String(selection).replace(/^\s+|\s+$/g,''))) {
        console.log(selection);
        var caretAt = getCaretPosition(selection, this);
        console.log(caretAt);
    }
});