#729 -- implementing reduceRight for object iteration

test/collections.js

@@ -119,7 +119,7 @@ $(document).ready(function() {
         object = {a: 1, b: 2},
         lastKey = _.keys(object).pop();
 
-    var expected = lastKey == 'b'
+    var expected = lastKey == 'a'
       ? [memo, 1, 'a', object]
       : [memo, 2, 'b', object];
 
@@ -131,13 +131,13 @@ $(document).ready(function() {
 
     // And again, with numeric keys.
 
-    object = {'2': 1, '1': 2};
+    object = {'2': 'a', '1': 'b'};
     lastKey = _.keys(object).pop();
     args = null;
 
-    expected = lastKey == 'b'
-      ? [memo, 1, '2', object]
-      : [memo, 2, '1', object];
+    expected = lastKey == '2'
+      ? [memo, 'a', '2', object]
+      : [memo, 'b', '1', object];
 
     _.reduceRight(object, function() {
       args || (args = _.toArray(arguments));

underscore.js

@@ -107,14 +107,18 @@
 
   // **Reduce** builds up a single result from a list of values, aka `inject`,
   // or `foldl`. Delegates to **ECMAScript 5**'s native `reduce` if available.
-  _.reduce = _.foldl = _.inject = function(obj, iterator, memo, context) {
+  _.reduce = _.foldl = _.inject = function(obj, iterator, memo, context, right) {
     var initial = arguments.length > 2;
     if (obj == null) obj = [];
-    if (nativeReduce && obj.reduce === nativeReduce) {
+    if (!right && nativeReduce && obj.reduce === nativeReduce) {
       if (context) iterator = _.bind(iterator, context);
       return initial ? obj.reduce(iterator, memo) : obj.reduce(iterator);
     }
     each(obj, function(value, index, list) {
+      if (right) {
+        index = right.keys[index];
+        list = right.list;
+      }
       if (!initial) {
         memo = value;
         initial = true;
@@ -134,9 +138,9 @@
       if (context) iterator = _.bind(iterator, context);
       return arguments.length > 2 ? obj.reduceRight(iterator, memo) : obj.reduceRight(iterator);
     }
-    // A right reduce on an object has undefined ordering, so we don't bother.
-    var reversed = _.isArray(obj) ? obj.reverse() : obj;
-    return _.reduce(reversed, iterator, memo, context);
+    var keys     = _.keys(obj).reverse();
+    var values   = _.toArray(obj).reverse();
+    return _.reduce(values, iterator, memo, context, {keys: keys, list: obj});
   };
 
   // Return the first value which passes a truth test. Aliased as `detect`.