Get an element's nth-child number in pure JavaScript

Question

I am making a function for my site where I set a data attribute which contains the nth-child number of that element.

My HTML markup:

<         


        

Answer 1

Check out this previous answer HERE.

It uses

var i = 0;
while( (child = child.previousSibling) != null ) 
  i++;
//at the end i will contain the index.