JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
  <li id="foo">foo</li>
  <li id="bar">bar</li>
  <li id="baz">baz</li>
</ul>

JavaScript

function indexInParent(node) {
        var children = node.parentNode.childNodes;
        var num = 0;
        for (var i=0; i<children.length; i++) {
             if (children[i]==node) return num;
             if (children[i].nodeType==1) num++;
        }
        return -1;
    }
console.log(indexInParent(document.getElementById('bar')));