JSFiddle - React, Tailwind, and code Playground

HTML

<div contenteditable="true">Please select some of this text and press a button below</div>

<span onclick="GetSelectedText()">Click</span>
<span onmousedown="GetSelectedText()">Mousedown</span>
<span unselectable="on" onclick="GetSelectedText()">Click, unselectable</span>

CSS

span {
    background-color: #ccc;
    padding: 3px;
    border: solid gray 1px;
    
}

*[unselectable="on"] {
   -moz-user-select: -moz-none;
   -khtml-user-select: none;
   -webkit-user-select: none;

   /*
     Introduced in IE 10.
     See http://ie.microsoft.com/testdrive/HTML5/msUserSelect/
   */
   -ms-user-select: none;
   user-select: none;
}

JavaScript

function GetSelectedText () {
    if (window.getSelection) {  // all browsers, except IE before version 9
        var range = window.getSelection ();
        alert (range.toString ());
    } 
    else {
        if (document.selection.createRange) { // Internet Explorer
            var range = document.selection.createRange ();
            alert (range.text);
        }
    }
}