// Click on any div element and see the output

​

document.querySelectorAll('.myClass').forEach((e) => { // Add the event listener to all the elements with class .myClass

  e.addEventListener('click', (event) => {

    var element = event.target;

    var index = Array.from(element 

      .parentNode // Get the parent node of the clicked element

      .querySelectorAll('.' + element.className)) // Select all the elements inside the parent node (siblings) with the same class name of the clicked element

      .indexOf(element) + 1; // Look for the index of the clicked element, + 1 

    console.log(index);

  });

});