jQuery's "when" and "then" all rolled up together.

jquery.ba-whenthen.js

/*!
 * jQuery whenthen - v0.1 - 3/12/2011
 * http://benalman.com/
 * 
 * Copyright (c) 2011 "Cowboy" Ben Alman
 * Dual licensed under the MIT and GPL licenses.
 * http://benalman.com/about/license/
 */

(function($){

  /*
    $.whenthen( deferred, [ done_callback, [ fail_callback ] ] )
    
    Pass a Deferred, `done` callback, and `fail` callback all at once. Multiple
    items of any arg can be specified by passing an array for that arg.
    
    Returns a promise.
  */
  $.whenthen = function( whens, dones, fails ) {
    var promise,
        methods = [$.when, 'done', 'fail'];

    $.each(arguments, function( i, arg ) {
      var method = i ? promise[methods[i]] : methods[i];
      promise = method[$.type(arg) === 'array' ? 'apply' : 'call'](promise, arg);
    });

    return promise;
  };

})(jQuery);

usage.js

/*
Sample Usage:

$.whenthen( deferred, function( a ) {
  // `a` are the value(s) the deferred was resolved with.
})

$.whenthen( array_of_two_deferreds, function( a, b ) {
  // `a` and `b` are the value(s) the deferreds were resolved with.
})

$.whenthen( deferred, function( a ) {
  // `a` are the value(s) the deferred was resolved with.
}, function( a ) {
  // `a` are the value(s) the deferred was rejected with.
})

... etc ...

*/

var dfds = [];
$.each("a b c d".split(" "),function( i, v ) {
  var dfd = $.Deferred();
  setTimeout(function(){
    if ( i === 3 ) {
      dfd.reject( "rejected " + i );
    } else {
      dfd.resolve( "resolved " + i );
    }
  }, 100);
  dfds.push(dfd);
});
var dfdYay = dfds[0],
    dfdYays = dfds.slice(0, -1),
    dfdNay = dfds[3],
    dfdNays = dfds.slice(1);

function logger( msg ) {
  return function() {
    console.log(msg, Array.prototype.slice.call(arguments));
  }
}

// The "jQuery" way.
$.when(dfds[0], dfds[1], dfds[2]).then(logger('jq done 1'), logger('jq fail (n/a)'));
$.when(dfds[0], dfds[1], dfds[2]).done(logger('jq done 2')).fail(logger('jq fail (n/a)'));
$.when(dfds[1], dfds[2], dfds[3]).then(logger('jq done (n/a)'), logger('jq fail 1'));
$.when(dfds[1], dfds[2], dfds[3]).done(logger('jq done (n/a)')).fail(logger('jq fail 2'));

// Options!
$.whenthen(dfdYay, logger('wt done 1'));
$.whenthen(dfdYay, [logger('wt done 2'), logger('wt done 3')]);
$.whenthen(dfdYays, [logger('wt done 4'), logger('wt done 5')]);
$.whenthen(dfdYay, logger('wt done 6'), logger('wt fail (n/a)'));
$.whenthen(dfdYays, logger('wt done 7'), [logger('wt fail (n/a)'), logger('wt fail (n/a)')]);
$.whenthen(dfdYay, [logger('wt done 8'), logger('wt done 9')], logger('wt fail (n/a)'));
$.whenthen(dfdYays, [logger('wt done 10'), logger('wt done 11')], [logger('wt fail (n/a)'), logger('wt fail (n/a)')]);

$.whenthen(dfdNay, logger('wt done (n/a)'));
$.whenthen(dfdNay, [logger('wt done (n/a)'), logger('wt done (n/a)')]);
$.whenthen(dfdNays, [logger('wt done (n/a)'), logger('wt done (n/a)')]);
$.whenthen(dfdNay, logger('wt done (n/a)'), logger('wt fail 1'));
$.whenthen(dfdNays, logger('wt done (n/a)'), [logger('wt fail 2'), logger('wt fail 3')]);
$.whenthen(dfdNay, [logger('wt done (n/a)'), logger('wt done (n/a)')], logger('wt fail 4'));
$.whenthen(dfdNays, [logger('wt done (n/a)'), logger('wt done (n/a)')], [logger('wt fail 5'), logger('wt fail 6')]);