如何让我的For循环将按钮的索引保存在另一个变量中？

var buttons = document.getElementsByTagName("button");
var indexArray = [];

for (var i = 0; i < buttons.length; i++) {
  (function(index) {
    buttons[index].addEventListener("click", function() {
      indexArray.push(index);
      console.log(indexArray);
    });
  })(i);
}

var buttons = document.getElementsByTagName("button");
var indexArray = [];

for (let i = 0; i < buttons.length; i++) {
  buttons[i].addEventListener("click", function() {
    indexArray.push(i);
    console.log(indexArray);
  });
}