jQuery index() in vanilla javascript

You can build your own function :

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;
}

Demonstration (open the console)

I've modified Travis J answer to not include TextNodes and made a function out of it.

You can run it in the console and see (on stackoverflow).

Classic way:

function getNodeindex( elm ){ 
    var c = elm.parentNode.children, 
        i = c.length;
        
    while(i--) 
      if( c[i] == elm ) 
        return i
}

With ES2015:

const getNodeindex = elm => [...elm.parentNode.children].findIndex(c => c == elm)

// or

const getNodeindex = elm => [...elm.parentNode.children].indexOf(elm)

Demo:

const getNodeindex = elm => [...elm.parentNode.children].indexOf(elm)

<button onclick="console.log(  getNodeindex(this)  )">get index</button>
<button onclick="console.log(  getNodeindex(this)  )">get index</button>
<button onclick="console.log(  getNodeindex(this)  )">get index</button>

I also want to point to another thread on the same matter, which has a great answer (for people seeking older IE support)

No loops needed, call Array#indexOf on .parentElement.children:

const element = document.querySelector('#baz');

[].indexOf.call(element.parentElement.children, element);
// => 2

You can even call it on a random list of elements, just like you can in jQuery:

const list = document.querySelectorAll('li');
const element = document.querySelector('#baz');

[].indexOf.call(list, element);
// => 2