JSFiddle - React, Tailwind, and code Playground

HTML

<textarea name="" id="texto" cols="30" rows="10">Texto de exemplo</textarea>

JavaScript

function showSelection(textComponent) {
    var selectedText;
    // IE version
    if (document.selection != undefined) {
        textComponent.focus();
        var sel = document.selection.createRange();
        selectedText = sel.text;
    }
    // Mozilla version
    else if (textComponent.selectionStart != undefined) {
        var startPos = textComponent.selectionStart;
        var endPos = textComponent.selectionEnd;
        selectedText = textComponent.value.substring(startPos, endPos)
    }
    return selectedText;
}

var textarea = document.getElementById('texto');
textarea.addEventListener('mouseup', function () {
    var textoSelecionado = showSelection(this);
    if (textoSelecionado) alert(textoSelecionado);
});