コナミコマンドを実装した

konami_command.js

var keyCodeCheck = (function () {
  var konamiCommandKeyCodes = [38, 38, 40, 40, 37, 39, 37, 39, 66, 65];
  var acceptIndex = 0;
  var timerId = null;

  var resetAcceptIndex = function () {
    console.log('最初からやりなおしです');
    acceptIndex = 0;
  };

  return function (e) {
    if (e.keyCode == konamiCommandKeyCodes[acceptIndex]) {
      clearTimeout(timerId);
      acceptIndex += 1;
      if (acceptIndex == konamiCommandKeyCodes.length) {
        alert('コナミコマンドを受け付けました');
        acceptIndex = 0;
      } else {
        timerId = setTimeout(resetAcceptIndex, 1000);
      }
    } else {
      acceptIndex = 0;
    }
  };
})();

window.onload = function () {
  document.body.addEventListener('keyup', keyCodeCheck);
};