This would be one way of doing it...

document.querySelectorAll will get an array of elements (buttons) based on the class selector.

We then loop over this array using forEach - button is the current element we are iterating over while index is the index of that element in the array.

Finally, while looping, we attach a "click" event listener to each button and when that event is fired, we console.log the index from earlier.

const buttons = document.querySelectorAll('.buttonEl')
buttons.forEach(function(button, index){
  button.addEventListener('click', function(){
    console.log(index)
  })
})

.action{
  display: flex;
  justify-content: space-around;
}

.buttonEl {
  background-color: #CCC;
  padding: 10px;
  border-radius: 5px;
}

<div class="action">
  <div class="buttonEl">Foo</div>
  <div class="buttonEl">Bar</div>
  <div class="buttonEl">Baz</div>
</div>

I believe you're looking for the indexOf array method.

// Arrays have the indexOf method, but NodeLists do not
const buttons = Array.from(document.querySelectorAll('.buttonEl'));
const buttonParent = document.querySelector('.action');
buttonParent.addEventListener('click', (evt) => {
  let target = evt.target;
  let index = buttons.indexOf(target);
  console.log(index);
});