JSFiddle - React, Tailwind, and code Playground

HTML

<div id="mainDiv" onmouseup="alert(getSelectionContainerElementId())">Select some content here. Here's <b id="bold">some bold text</b></div>

JavaScript

function getSelectionContainerElementId() {
    var sel = window.getSelection();
    if (sel.rangeCount > 0) {
        var range = sel.getRangeAt(0);
        var container = range.commonAncestorContainer;

        // container could be a text node, so use its parent if so
        if (container.nodeType == 3) {
            container = container.parentNode;
        }
        return container.id;
    }
    return null;
}