get index of child with event.currentTarget

Question

How to get rid of jQuery's ".index()" here? All I am looking for is a native way to get the index of the button clicked, see source code below (works perfectly but I really couldn't find any Vanilla-JS solution for this and I don't want to use jQuery for such a small task). Thanks.

    function navButtonClick(ev) {
        var buttonIndex = $(ev.currentTarget).index(); /* How to get rid of jQuery? */

        document.getElementById("output").innerHTML = buttonIndex;
    }

    function startTemplate() {
        var i;
        var navItems = document.querySelectorAll("#navigation button");

        for (i=0; i < navItems.length; i++) {
            navItems[i].addEventListener("click", navButtonClick);
        }
    }

    document.addEventListener("DOMContentLoaded", startTemplate);
    

The HTML part looks like this:

    <nav id="navigation" role="navigation">
        <button type="button"> <em>Navi tab 1</em> </button>
        <button type="button"> Navi tab 2 </button>
        <button type="button"> Navi tab 3 </button>
        <button type="button"> Navi tab 4 </button>
    </nav>

    <div id="output"></div>

Answer 1

You can do this in two different ways. First, pass the index of the buttons to the corresponding function while adding the event listeners:

function navButtonClick(buttonIndex) {
    document.getElementById("output").innerHTML = buttonIndex;
}

function startTemplate() {
    var i;
    var navItems = document.querySelectorAll("#navigation button");
    for (i = 0; i < navItems.length; i++) {
        navItems[i].addEventListener("click", navButtonClick.bind(null, i));
    }
}

startTemplate();

JSFiddle

Second, create a function which will dynamically check the index of the element:

function navButtonClick() {
    document.getElementById("output").innerHTML = getIndex(this);
}

function getIndex(elm) {
  var parent = elm.parentElement;
  for (var i = 0; i < parent.children.length; i++) {
    if (parent.children[i].isEqualNode(elm)) {
        return i;
    }
  }
}

function startTemplate() {
    var i;
    var navItems = document.querySelectorAll("#navigation button");
    for (i = 0; i < navItems.length; i++) {
        navItems[i].addEventListener("click", navButtonClick);
    }
}

startTemplate();

JSFiddle