A delegate is a helper object used by another object. The delegator may send the delegate certain messages, and provide a default implementation when there is no delegate or the delegate does not respond to a message. This pattern is heavily used in Cocoa framework on Mac OS X. See also wp:Delegation pattern.

Objects responsibilities:

Delegator:

 Keep an optional delegate instance.
 Implement "operation" method, returning the delegate "thing" if the delegate respond to "thing", or the string "default implementation".
Delegate:

 Implement "thing" and return the string "delegate implementation"
Show how objects are created and used. First, without a delegate, then with a delegate that does not implement "thing", and last with a delegate that implements "thing".

 
class Delegator
  operation: ->
    if @delegate and typeof (@delegate.thing) is "function"
      return @delegate.thing() 
    "default implementation"
 
class Delegate
  thing: ->
    "Delegate Implementation"
 
testDelegator = ->
  # Delegator with no delegate.
  a = new Delegator()
  console.log a.operation()
 
  # Delegator with delegate not implementing "thing"
  a.delegate = "A delegate may be any object"
  console.log a.operation()
 
  # Delegator with delegate that does implement "thing"
  a.delegate = new Delegate()
  console.log a.operation()
 
testDelegator()
 