Merge pull request #3 from bjrmatos/master

make it more exactly like bluebird's Promise.reduce

index.js

@@ -11,9 +11,20 @@ function reduce(fn, start) {
   assert.equal(typeof fn, 'function')
   return function(val) {
     val = Array.isArray(val) ? val : [val]
-    return val.reduce(function (promise, curr) {
+
+    const length = val.length;
+
+    if (length === 0) {
+      return Promise.resolve(start);
+    }
+
+    return val.reduce(function (promise, curr, index, arr) {
       return promise.then(function (prev) {
-        return fn(prev, curr)
+        if (prev === undefined && length === 1) {
+          return curr;
+        }
+
+        return fn(prev, curr, index, arr)
       })
     }, Promise.resolve(start))
   }

test.js

@@ -7,7 +7,7 @@ test('promise-reduce should assert input types', function (t) {
   t.throws(reduce.bind(null, 123))
 })
 
-test('promise-reuduce should accept a single val', function (t) {
+test('promise-reduce should accept a single val', function (t) {
   t.plan(1)
 
   const res = Promise.resolve(2)
@@ -38,6 +38,30 @@ test('promise-reduce should reduce a fn', function (t) {
   }
 })
 
+test('should pass reducer arguments to callback', function(t) {
+  const arrTest = [1, 2]
+
+  const res = Promise.resolve(arrTest)
+    .then(reduce(reduceFn, 0))
+    .then(checkFn)
+
+  function reduceFn(prev, next, index, arr) {
+    t.equal(4, arguments.length)
+    t.equal(arrTest, arr)
+
+    return new Promise(function (resolve) {
+      setTimeout(function () {
+        resolve(prev + next)
+      }, 1)
+    })
+  }
+
+  function checkFn(val) {
+    t.equal(3, val)
+    t.end()
+  }
+})
+
 test('should not continue until last iteration has been resolved', function (t) {
   t.plan(1)
   const res = Promise.resolve([1, 2, 3])
@@ -56,3 +80,25 @@ test('should not continue until last iteration has been resolved', function (t)
     t.equal(6, val)
   }
 })
+
+test('should not call callback when initial value is undefined and iterable contains one item', function (t) {
+  t.plan(1)
+  const res = Promise.resolve([1])
+    .then(reduce(function() {}, undefined))
+    .then(checkFn)
+
+  function checkFn(val) {
+    t.equal(1, val, 'should return the item in iterable')
+  }
+})
+
+test('should not call callback when iterable is empty', function (t) {
+  t.plan(1)
+  const res = Promise.resolve([])
+    .then(reduce(function() {}, 10))
+    .then(checkFn)
+
+  function checkFn(val) {
+    t.equal(10, val, 'should return initial value')
+  }
+})