From 6fbc797d1ea42272ff929e295e872b0625b15d48 Mon Sep 17 00:00:00 2001
From: Graeme Yeates <yeatesgraeme@gmail.com>
Date: Mon, 24 Aug 2015 18:00:41 -0400
Subject: Add async.transform

---
 lib/async.js       | 16 +++++++++++++++-
 test/test-async.js | 53 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+), 1 deletion(-)

diff --git a/lib/async.js b/lib/async.js
index 1f1e0ef..f8d9019 100644
--- a/lib/async.js
+++ b/lib/async.js
@@ -379,7 +379,7 @@
                 callback(err);
             });
         }, function (err) {
-            callback(err || null, memo);
+            callback(err, memo);
         });
     };
 
@@ -389,6 +389,20 @@
         async.reduce(reversed, memo, iterator, callback);
     };
 
+    async.transform = function (arr, memo, iterator, callback) {
+        if (arguments.length === 3) {
+            callback = iterator;
+            iterator = memo;
+            memo = _isArray(arr) ? [] : {};
+        }
+
+        async.eachOf(arr, function(v, k, cb) {
+            iterator(memo, v, k, cb);
+        }, function(err) {
+            callback(err, memo);
+        });
+    };
+
     function _filter(eachfn, arr, iterator, callback) {
         var results = [];
         eachfn(arr, function (x, index, callback) {
diff --git a/test/test-async.js b/test/test-async.js
index 18894b0..a6af0e4 100755
--- a/test/test-async.js
+++ b/test/test-async.js
@@ -2023,6 +2023,59 @@ exports['foldr alias'] = function(test){
     test.done();
 };
 
+exports['transform implictly determines memo if not provided'] = function(test){
+    async.transform([1,2,3], function(memo, x, v, callback){
+        memo.push(x + 1);
+        callback();
+    }, function(err, result){
+        test.same(result, [2, 3, 4]);
+        test.done();
+    });
+};
+
+exports['transform async with object memo'] = function(test){
+    test.expect(2);
+
+    async.transform([1,3,2], {}, function(memo, v, k, callback){
+        setTimeout(function() {
+            memo[k] = v;
+            callback();
+        });
+    }, function(err, result) {
+        test.equals(err, null);
+        test.same(result, {
+            0: 1,
+            1: 3,
+            2: 2
+        });
+        test.done();
+    });
+};
+
+exports['transform iterating object'] = function(test){
+    test.expect(2);
+
+    async.transform({a: 1, b: 3, c: 2}, function(memo, v, k, callback){
+        setTimeout(function() {
+            memo[k] = v + 1;
+            callback();
+        });
+    }, function(err, result) {
+        test.equals(err, null);
+        test.same(result, {a: 2, b: 4, c: 3});
+        test.done();
+    });
+};
+
+exports['transform error'] = function(test){
+    async.transform([1,2,3], function(a, v, k, callback){
+        callback('error');
+    }, function(err){
+        test.equals(err, 'error');
+        test.done();
+    });
+};
+
 exports['filter'] = function(test){
     async.filter([3,1,2], filterIterator, function(results){
         test.same(results, [3,1]);
-- 
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