Determine if a modifier key is pressed in JavaScript.

modifierKey.js

// modifierKey used to check if cmd+click, shift+click, etc. 
!function($, global){
  var $doc = $(document);
  var keys;

  global.modifierKey = false;

   global.keys = keys = {
      'UP': 38,
      'DOWN': 40,
      'LEFT': 37,
      'RIGHT': 39,
      'RETURN': 13,
      'ESCAPE': 27,
      'BACKSPACE': 8,
      'SPACE': 32
  };

  // borrowed from Galleria.js
  var keyboard = {
    map: {},
    bound: false,

    press: function(e) {
      var key = e.keyCode || e.which;
      if ( key in keyboard.map && typeof keyboard.map[key] === 'function' ) {
        keyboard.map[key].call(self, e);
      }
    },

    attach: function(map){
      var key, up;

      for(key in map) {
        if (map.hasOwnProperty(key)) {
          up = key.toUpperCase();
          if (up in keyboard.keys) {
            keyboard.map[keyboard.keys[up]] = map[key];
          } else {
            keyboard.map[up] = map[key];
          }
        }
      }
      if (!keyboard.bound) {
        keyboard.bound = true;
        $doc.bind('keydown', keyboard.press);
      }
    },

    detach: function() {
      keyboard.bound = false;
      keyboard.map = {};
      $doc.unbind('keydown', keyboard.press);
    }
  };

  $doc.keydown(function(e) {
    var key = e.keyCode || e.which;
    if (key === 16 || key === 91 || key === 18 || key === 17) {
      modifierKey = true;
    } else {
      modifierKey = false;
    }
  });

  $doc.keyup(function(e) {
    modifierKey = false;
  });
}(jQuery, window);