Remove reduce error throwing

Closes jashkenas#1858.

test/collections.js

@@ -122,9 +122,7 @@
     ok(_.reduce(null, _.noop, 138) === 138, 'handles a null (with initial value) properly');
     equal(_.reduce([], _.noop, undefined), undefined, 'undefined can be passed as a special case');
     equal(_.reduce([_], _.noop), _, 'collection of length one with no initial value returns the first item');
-
-    throws(function() { _.reduce([], _.noop); }, TypeError, 'throws an error for empty arrays with no initial value');
-    throws(function() {_.reduce(null, _.noop);}, TypeError, 'handles a null (without initial value) properly');
+    equal(_.reduce([], _.noop), undefined, 'returns undefined when collection is empty and no initial value');
   });
 
   test('foldl', function() {
@@ -145,9 +143,7 @@
     equal(_.reduceRight([_], _.noop), _, 'collection of length one with no initial value returns the first item');
 
     equal(_.reduceRight([], _.noop, undefined), undefined, 'undefined can be passed as a special case');
-
-    throws(function() { _.reduceRight([], _.noop); }, TypeError, 'throws an error for empty arrays with no initial value');
-    throws(function() {_.reduceRight(null, _.noop);}, TypeError, 'handles a null (without initial value) properly');
+    equal(_.reduceRight([], _.noop), undefined, 'returns undefined when collection is empty and no initial value');
 
     // Assert that the correct arguments are being passed.
 

underscore.js

@@ -160,8 +160,6 @@
     return results;
   };
 
-  var reduceError = 'Reduce of empty array with no initial value';
-
   // **Reduce** builds up a single result from a list of values, aka `inject`,
   // or `foldl`.
   _.reduce = _.foldl = _.inject = function(obj, iteratee, memo, context) {
@@ -171,7 +169,6 @@
         length = (keys || obj).length,
         index = 0, currentKey;
     if (arguments.length < 3) {
-      if (!length) throw new TypeError(reduceError);
       memo = obj[keys ? keys[index++] : index++];
     }
     for (; index < length; index++) {
@@ -189,10 +186,9 @@
         index = (keys || obj).length,
         currentKey;
     if (arguments.length < 3) {
-      if (!index) throw new TypeError(reduceError);
       memo = obj[keys ? keys[--index] : --index];
     }
-    while (index--) {
+    while (index-- > 0) {
       currentKey = keys ? keys[index] : index;
       memo = iteratee(memo, obj[currentKey], currentKey, obj);
     }