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".

function Delegator() {
  this.delegate = null ;
  this.operation = function(){
    if(this.delegate && typeof(this.delegate.thing) == 'function')
      return this.delegate.thing() ;
    return 'default implementation' ;
  }
}
 
function Delegate() {
  this.thing = function(){
    return 'Delegate Implementation' ;
  }
}
 
function testDelegator(){
  var a = new Delegator() ;
  document.write(a.operation() + "\n") ;
 
  a.delegate = 'A delegate may be any object' ; 
  document.write(a.operation() + "\n") ;
 
  a.delegate = new Delegate() ;
  document.write(a.operation() + "\n") ;
}