let.js

const toArray = iterables => {
  var res = [];
  for (var i = 0; i < iterables.length; i++) {
    res[i] = iterables[i];
  }
  return res;
}

const query = selector => toArray(document.querySelectorAll(selector));

query('.btn').forEach((button, i) => {
  button.innerText = i + 1;
});