Implements joined jQuery.when statements. Makes it so calling jQuery.when with no parameter returns a resolved promise. Ensures promise method on promises supports the promise(obj) signature. Ensures a deferred and its promise always return the same promise (itself for the promise). Unit tests provided.

src/core.js

@@ -898,9 +898,10 @@ jQuery.extend({
 	Deferred: function( func ) {
 
 		var deferred = jQuery._Deferred(),
-			failDeferred = jQuery._Deferred();
+			failDeferred = jQuery._Deferred(),
+			promise;
 
-		// Add errorDeferred methods and redefine cancel
+		// Add errorDeferred methods, then and promise
 		jQuery.extend( deferred , {
 
 			then: function( doneCallbacks , failCallbacks ) {
@@ -914,13 +915,15 @@ jQuery.extend({
 			// Get a promise for this deferred
 			// If obj is provided, the promise aspect is added to the object
 			promise: function( obj ) {
-				obj = obj || {};
+				if ( obj == null ) {
-				jQuery.each( "then done fail isResolved isRejected".split( " " ) , function( _ , method ) {
+					if ( promise ) {
+						return promise;
+					}
+					promise = obj = {};
+				}
+				jQuery.each( "then done fail isResolved isRejected promise".split( " " ) , function( _ , method ) {
 					obj[ method ] = deferred[ method ];
 				});
-				obj.promise = function() {
-					return obj;
-				};
 				return obj;
 			}
 
@@ -942,10 +945,32 @@ jQuery.extend({
 
 	// Deferred helper
 	when: function( object ) {
-		object = object && jQuery.isFunction( object.promise ) ?
+		var args = arguments,
+			length = args.length,
+			deferred = length <= 1 && object && jQuery.isFunction( object.promise ) ?
 				object :
-			jQuery.Deferred().resolve( object );
+				jQuery.Deferred(),
-		return object.promise();
+			promise = deferred.promise(),
+			resolveArray;
+
+		if ( length > 1 ) {
+			resolveArray = new Array( length );
+			jQuery.each( args, function( index, element, args ) {
+				jQuery.when( element ).done( function( value ) {
+					args = arguments;
+					resolveArray[ index ] = args.length > 1 ? slice.call( args , 0 ) : value;
+					if( ! --length ) {
+						deferred.fire( promise, resolveArray );
+					}
+				}).fail( function() {
+					deferred.fireReject( promise, arguments );
+				});
+				return !deferred.isRejected();
+			});
+		} else if ( deferred !== object ) {
+			deferred.resolve( object );
+		}
+		return promise;
 	},
 
 	// Use of jQuery.browser is frowned upon.

test/unit/core.js

@@ -21,7 +21,7 @@ test("jQuery()", function() {
 	equals( jQuery(null).length, 0, "jQuery(null) === jQuery([])" );
 	equals( jQuery("").length, 0, "jQuery('') === jQuery([])" );
 
-	var obj = jQuery("div")
+	var obj = jQuery("div");
 	equals( jQuery(obj).selector, "div", "jQuery(jQueryObj) == jQueryObj" );
 
 		// can actually yield more than one, when iframes are included, the window is an array as well
@@ -1003,7 +1003,7 @@ test("jQuery._Deferred()", function() {
 
 test("jQuery.Deferred()", function() {
 
-	expect( 4 );
+	expect( 6 );
 
 	jQuery.Deferred( function( defer ) {
 		strictEqual( this , defer , "Defer passed as this & first argument" );
@@ -1023,11 +1023,16 @@ test("jQuery.Deferred()", function() {
 	}, function() {
 		ok( true , "Error on reject" );
 	});
+
+	var tmp = jQuery.Deferred();
+
+	strictEqual( tmp.promise() , tmp.promise() , "Test deferred always return same promise" );
+	strictEqual( tmp.promise() , tmp.promise().promise() , "Test deferred's promise always return same promise as deferred" );
 });
 
 test("jQuery.when()", function() {
 
-	expect( 21 );
+	expect( 23 );
 
 	// Some other objects
 	jQuery.each( {
@@ -1050,6 +1055,10 @@ test("jQuery.when()", function() {
 
 	} );
 
+	ok( jQuery.isFunction( jQuery.when().then( function( resolveValue ) {
+		strictEqual( resolveValue , undefined , "Test the promise was resolved with no parameter" );
+	} ).promise ) , "Test calling when with no parameter triggers the creation of a new Promise" );
+
 	var cache, i;
 
 	for( i = 1 ; i < 4 ; i++ ) {
@@ -1064,6 +1073,37 @@ test("jQuery.when()", function() {
 	}
 });
 
+test("jQuery.when() - joined", function() {
+
+	expect(8);
+
+	jQuery.when( 1, 2, 3 ).done( function( a, b, c ) {
+		strictEqual( a , 1 , "Test first param is first resolved value - non-observables" );
+		strictEqual( b , 2 , "Test second param is second resolved value - non-observables" );
+		strictEqual( c , 3 , "Test third param is third resolved value - non-observables" );
+	}).fail( function() {
+		ok( false , "Test the created deferred was resolved - non-observables");
+	});
+
+	var successDeferred = jQuery.Deferred().resolve( 1 , 2 , 3 ),
+		errorDeferred = jQuery.Deferred().reject( "error" , "errorParam" );
+
+	jQuery.when( 1 , successDeferred , 3 ).done( function( a, b, c ) {
+		strictEqual( a , 1 , "Test first param is first resolved value - resolved observable" );
+		same( b , [ 1 , 2 , 3 ] , "Test second param is second resolved value - resolved observable" );
+		strictEqual( c , 3 , "Test third param is third resolved value - resolved observable" );
+	}).fail( function() {
+		ok( false , "Test the created deferred was resolved - resolved observable");
+	});
+
+	jQuery.when( 1 , errorDeferred , 3 ).done( function() {
+		ok( false , "Test the created deferred was rejected - rejected observable");
+	}).fail( function( error , errorParam ) {
+		strictEqual( error , "error" , "Test first param is first rejected value - rejected observable" );
+		strictEqual( errorParam , "errorParam" , "Test second param is second rejected value - rejected observable" );
+	});
+});
+
 test("jQuery.subclass", function(){
 	expect(378);