JSFiddle - React, Tailwind, and code Playground

HTML

Highlight some text with mouse in this container:
<div id="test">
asjdiajsidjasidjisadjai aijdiasjda djiasjd iadj asid
</div>
Result div:
<div id="result"></div>

CSS

#test{ padding: 10px; border: solid 1px; }
#result { padding: 10px; border: solid 1px green; }

JavaScript

const testDiv = document.getElementById('test');
const resultDiv = document.getElementById('result');

function getSelectionText() {
    var text = "";
    if (window.getSelection) {
        text = window.getSelection().toString();
    } else if (document.selection && document.selection.type != "Control") {
        text = document.selection.createRange().text;
    }
    return text;
}

testDiv.addEventListener('mouseup', function(e) {
	const selectedText = getSelectionText();
	resultDiv.textContent = selectedText;
})