adding a reduceRight (it's in JS 1.8), and aliasing foldl and foldr

index.html

@@ -123,10 +123,14 @@ <h2>Table of Contents</h2>
       <b>Collections</b>
       <br /> 
       <span class="methods"><a href="#each">each</a>, <a href="#map">map</a>, 
-      <a href="#reduce">reduce</a>, <a href="#detect">detect</a>, <a href="#select">select</a>, <a href="#reject">reject</a>, <a href="#all">all</a>, 
-      <a href="#any">any</a>, <a href="#include">include</a>, <a href="#invoke">invoke</a>, <a href="#pluck">pluck</a>, <a href="#max">max</a>, 
-      <a href="#min">min</a>, <a href="#sortBy">sortBy</a>, <a href="#sortedIndex">sortedIndex</a>, <a href="#toArray">toArray</a>, 
-      <a href="#size">size</a></span>
+      <a href="#reduce">reduce</a>, <a href="#reduceRight">reduceRight</a>, 
+      <a href="#detect">detect</a>, <a href="#select">select</a>, 
+      <a href="#reject">reject</a>, <a href="#all">all</a>, 
+      <a href="#any">any</a>, <a href="#include">include</a>, 
+      <a href="#invoke">invoke</a>, <a href="#pluck">pluck</a>, 
+      <a href="#max">max</a>, <a href="#min">min</a>, 
+      <a href="#sortBy">sortBy</a>, <a href="#sortedIndex">sortedIndex</a>, 
+      <a href="#toArray">toArray</a>, <a href="#size">size</a></span>
     </p>
 
     <p>
@@ -195,7 +199,7 @@ <h2>Collection Functions (Arrays or Objects)</h2>
 
       <p id="reduce">
         <b class="header">reduce</b><code>_.reduce(list, memo, iterator, [context])</code>
-        <span class="alias">Alias: <b>inject</b></span>
+        <span class="alias">Aliases: <b>inject, foldl</b></span>
         <br />
         Also known as <b>inject</b> and <b>foldl</b>, <b>reduce</b> boils down a 
         <b>list</b> of values into a single value. <b>Memo</b> is the initial state
@@ -205,6 +209,19 @@ <h2>Collection Functions (Arrays or Objects)</h2>
       <pre>
 var sum = _.reduce([1, 2, 3], 0, function(memo, num){ return memo + num });
 =&gt; 6
+</pre>
+
+      <p id="reduceRight">
+        <b class="header">reduceRight</b><code>_.reduceRight(list, memo, iterator, [context])</code>
+        <span class="alias">Alias: <b>foldr</b></span>
+        <br />
+        The right-associative version of <b>reduce</b>. Delegates to the 
+        JavaScript 1.8 version of <b>reduceRight</b>, if it exists.
+      </p>
+      <pre>
+var list = [[0, 1], [2, 3], [4, 5]];
+var flat = _.reduceRight(list, [], function(a, b) { return a.concat(b); });
+=&gt; [4, 5, 2, 3, 0, 1]
 </pre>
 
       <p id="detect">

test/collections.js

@@ -47,10 +47,19 @@ $(document).ready(function() {
     var sum = _.reduce([1, 2, 3], 0, function(sum, num){ return sum + num; });
     equals(sum, 6, 'can sum up an array');
 
+    var context = {multiplier : 3};
+    sum = _.reduce([1, 2, 3], 0, function(sum, num){ return sum + num * this.multiplier; }, context);
+    equals(sum, 18, 'can reduce with a context object');
+
     sum = _.inject([1, 2, 3], 0, function(sum, num){ return sum + num; });
     equals(sum, 6, 'aliased as "inject"');
   });
 
+  test('collections: reduceRight', function() {
+    var list = _.foldr([1, 2, 3], '', function(memo, num){ return memo + num; });
+    equals(list, '321', 'can perform right folds');
+  });
+
   test('collections: detect', function() {
     var result = _.detect([1, 2, 3], function(num){ return num * 2 == 4; });
     equals(result, 2, 'found the first "2" and broke the loop');

underscore.js

@@ -61,14 +61,26 @@
   };
 
   // Reduce builds up a single result from a list of values. Also known as
-  // inject, or foldl.
+  // inject, or foldl. Uses JavaScript 1.8's version of reduce, if possible.
   _.reduce = function(obj, memo, iterator, context) {
+    if (obj && obj.reduce) return obj.reduce(_.bind(iterator, context), memo);
     _.each(obj, function(value, index, list) {
       memo = iterator.call(context, memo, value, index, list);
     });
     return memo;
   };
 
+  // The right-associative version of reduce, also known as foldr. Uses 
+  // JavaScript 1.8's version of reduceRight, if available.
+  _.reduceRight = function(obj, memo, iterator, context) {
+    if (obj && obj.reduceRight) return obj.reduceRight(_.bind(iterator, context), memo);
+    var reversed = _.clone(_.toArray(obj)).reverse();
+    _.each(reversed, function(value, index) {
+      memo = iterator.call(context, memo, value, index, obj);
+    });
+    return memo;
+  };
+
   // Return the first value which passes a truth test.
   _.detect = function(obj, iterator, context) {
     var result;
@@ -368,6 +380,7 @@
 
   // Create a (shallow-cloned) duplicate of an object.
   _.clone = function(obj) {
+    if (_.isArray(obj)) return obj.slice(0);
     return _.extend({}, obj);
   };
 
@@ -455,7 +468,8 @@
   /*------------------------------- Aliases ----------------------------------*/
 
   _.forEach  = _.each;
-  _.inject   = _.reduce;
+  _.foldl    = _.inject       = _.reduce;
+  _.foldr    = _.reduceRight;
   _.filter   = _.select;
   _.every    = _.all;
   _.some     = _.any;