const onClick = event => {
  // use Event object to get a reference 
  // to the button that was clicked
  const element = event.target
  
  // use the "dataset" API to read/write
  // data into the element
  const data = element.dataset
  
  // try to read the current counter value
  // from the element's data (new API)
  // with a ternary conditional operator.
  //
  // NOTE: element's data are always strings
  // so we need to convert it to a number
  //
  // In case there is not "count" available
  // it means it is the first click, so
  // we instanciate a default value of "1".
  // This is usually called "base condition"
  const count = data.count
    ? Number(data.count)
    : 1
  
  // Update the button's text with the
  // counter value:
  element.innerHTML = `click count: ${count}`
  
  // Write back the new counter value to
  // the element, so next click it will be
  // correctly remembered and displayed
  data.count = count + 1
}

const addOnClick = element =>
  element.addEventListener('click', onClick)

document
  .querySelectorAll('.btn')
  .forEach(addOnClick)