Array.prototype.{reduce, reduceRight}: Wrong order of operations when determining initial value.

src/array.js

Index: src/array.js
diff --git a/src/array.js b/src/array.js
index 44deff7de42c60986741615b1be389f9668dbf85..d7ae4ec76d3aefe593f43f58914f786abbb00ef0 100644
--- a/src/array.js
+++ b/src/array.js
@@ -1394,13 +1394,15 @@ function ArrayReduce(callback, current) {
   if (!IS_SPEC_FUNCTION(callback)) {
     throw MakeTypeError('called_non_callable', [callback]);
   }
+  if (length == 0 && IS_UNDEFINED(current)) {
+    throw MakeTypeError('reduce_no_initial', []);
+  }

[wingo, 2014/10/07 11:04:31]

I think the test should be %_ArgumentsLength() < 2 instead of
IS_UNDEFINED(current); explicitly passing undefined is fine.

[arv (Not doing code reviews), 2014/10/07 14:16:02]

Yup. The spec says: "If initialValue is present, then" so an args length check
is needed.

https://people.mozilla.org/~jorendorff/es6-draft.html#sec-array.prototype.reduce

[Diego Pino, 2014/10/09 10:18:57]

What I intended to code with that line was step 7 of the algorithm.

7. If len is 0 and initialValue is not present, throw a TypeError exception.
(https://people.mozilla.org/~jorendorff/es6-draft.html#sec-array.prototype.reduce).

However I realized is not needed actually, because the code below that searches
for initialValue, when is not defined (%_ArgumentsLength < 2), covers that case
too. There's already a test for this (array length equals 0 and current equals
undefined).

 
   var i = 0;
   find_initial: if (%_ArgumentsLength() < 2) {
     for (; i < length; i++) {
-      current = array[i];
-      if (!IS_UNDEFINED(current) || i in array) {
-        i++;
+      if (i in array) {
+        current = array[i++];
         break find_initial;
       }
     }
@@ -1431,13 +1433,15 @@ function ArrayReduceRight(callback, current) {
   if (!IS_SPEC_FUNCTION(callback)) {
     throw MakeTypeError('called_non_callable', [callback]);
   }
+  if (length == 0 && IS_UNDEFINED(current)) {
+    throw MakeTypeError('reduce_no_initial', []);
+  }

[wingo, 2014/10/07 11:04:31]

likewise

[arv (Not doing code reviews), 2014/10/07 14:16:02]

Yup.
https://people.mozilla.org/~jorendorff/es6-draft.html#sec-array.prototype.red...

 
   var i = length - 1;
   find_initial: if (%_ArgumentsLength() < 2) {
     for (; i >= 0; i--) {
-      current = array[i];
-      if (!IS_UNDEFINED(current) || i in array) {
-        i--;
+      if (i in array) {
+        current = array[i--];
         break find_initial;
       }
     }