How to detect whether a hash change came from the Back or Forward button

back_forward.js

var detectBackOrForward = function(onBack, onForward) {
  hashHistory = [window.location.hash];
  historyLength = window.history.length;

  return function() {
    var hash = window.location.hash, length = window.history.length;
    if (hashHistory.length && historyLength == length) {
      if (hashHistory[hashHistory.length - 2] == hash) {
        hashHistory = hashHistory.slice(0, -1);
        onBack();
      } else {
        hashHistory.push(hash);
        onForward();
      }
    } else {
      hashHistory.push(hash);
      historyLength = length;
    }
  }
};

window.addEventListener("hashchange", detectBackOrForward(
  function() { console.log("back") },
  function() { console.log("forward") }
));