Copying loop iterator

HTML

<span id="buttons"></span>

JavaScript

var buttons = document.getElementById("buttons");
for (var i = 0; i < 5; i++) {
  var btn = document.createElement("button");
  btn.onclick = function(index) {
    alert(index);
  }.bind(null, i);
  btn.innerHTML = "Button " + i;
  buttons.appendChild(btn);
}