Clever pattern by Ryan Stout to get the opposite value for strings.

var oppositeOf = {
  'up' : 'down',
  'down' : 'up',
  'left' : 'right',
  'right' : 'left',
  'black' : 'white',
  'white' : 'black'
};

oppositeOf['left'];
// => 'right'

Phil Dokas:

…That’s the memento design pattern. That’s how Undo in OS X is implemented :)