JSFiddle - React, Tailwind, and code Playground

HTML

<ul id="pagination">
            <li onclick="GetIndex(this)">One</li>
            <li onclick="GetIndex(this)">Two</li>
            <li onclick="GetIndex(this)">Three</li>
            <li onclick="GetIndex(this)">Four</li>
        </ul>

CSS

#pagination li { 
    background: pink;
    display: block;
    border: 1px solid #fff;
}

JavaScript

var pagination = document.getElementById('pagination');

function GetIndex(pagination)
{   
    var aElements = pagination.parentNode.getElementsByTagName("li");
    var aElementsLength = aElements.length;

    var index;
    for (var i = 0; i < aElementsLength; i++)
    {
        if (aElements[i] == pagination)
        {
            index = i;
            alert(index);
            return index;
        }
    }
}