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

jquery.ba-whenthen.js

/*!
 * jQuery whenthen - v0.2 - 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.
    
    Note: when passing a single deferred, if you are unsure whether it is a
    promise or a single value that might be an array, you must disambiguate it
    by surrounding it with [] (thus explicitly passing it as an array).
  */

  $.whenthen = function( whens, dones, fails ) {
    var promise;

    $.each(arguments, function( i, arg ) {
      var method = i ? promise[i === 1 ? 'done' : 'fail'] : $.when;
      promise = method[$.type(arg) === 'array' ? 'apply' : 'call'](promise, arg);
    });

    return promise;
  };

})(jQuery);

unit.js

// Unit tests

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),
    testArr = [],
    testSuccess = 0;

function test( val ) {
  if ( val ) {
    testSuccess++;
  }
  return function() {
    testArr.push(val);
  }
}

setTimeout(function(){
  console.log(testArr.length === testSuccess && testArr.indexOf(false) < 0 ? 'SUCCESS' : 'FAILURE');
}, 200);


// The "jQuery" way.
$.when(dfds[0], dfds[1], dfds[2]).then(test(true), test(false));
$.when(dfds[0], dfds[1], dfds[2]).done(test(true)).fail(test(false));
$.when(dfds[1], dfds[2], dfds[3]).then(test(false), test(true));
$.when(dfds[1], dfds[2], dfds[3]).done(test(false)).fail(test(true));

// Options!
$.whenthen(dfdYay, test(true));
$.whenthen(dfdYay, [test(true), test(true)]);
$.whenthen(dfdYays, [test(true), test(true)]);
$.whenthen(dfdYay, test(true), test(false));
$.whenthen(dfdYays, test(true), [test(false), test(false)]);
$.whenthen(dfdYay, [test(true), test(true)], test(false));
$.whenthen(dfdYays, [test(true), test(true)], [test(false), test(false)]);

$.whenthen(dfdNay, test(false));
$.whenthen(dfdNay, [test(false), test(false)]);
$.whenthen(dfdNays, [test(false), test(false)]);
$.whenthen(dfdNay, test(false), test(true));
$.whenthen(dfdNays, test(false), [test(true), test(true)]);
$.whenthen(dfdNay, [test(false), test(false)], test(true));
$.whenthen(dfdNays, [test(false), test(false)], [test(true), test(true)]);

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...