Fix reduce

DzyubSpirit · nechaido · commit 6f76064b591b · 2019-02-06T12:27:53.000+02:00
* Fix invalid behavior when an empty array with no initial value is passed * Rename error message constant to better represent its meaning * Add missing documentation PR-URL: #406
diff --git a/lib/array.js b/lib/array.js
@@ -151,7 +151,8 @@ const filter = (items, fn, done) => {
   }
 };
 
-const EMPTY_ARR = 'Metasync: reduce of empty array with no initial value';
+const REDUCE_EMPTY_ARR =
+  'Metasync: reduce of empty array with no initial value';
 
 // Asynchronous reduce
 //   items - <Array>, incoming
@@ -160,6 +161,8 @@ const EMPTY_ARR = 'Metasync: reduce of empty array with no initial value';
 //     current - <any>, current element being processed in the array
 //     callback - <Function>, callback for returning value
 //         back to reduce function
+//       err - <Error> | <null>
+//       data - <any>, resulting value
 //     counter - <number>, index of the current element
 //         being processed in array
 //     items - <Array>, the array reduce was called upon
@@ -171,15 +174,20 @@ const EMPTY_ARR = 'Metasync: reduce of empty array with no initial value';
 const reduce = (items, fn, done, initial) => {
   done = done || common.emptyness;
   const len = items.length;
-  let count = typeof initial === 'undefined' ? 1 : 0;
+  const hasInitial = typeof initial !== 'undefined';
 
-  if (!len) {
-    const err = count ? new TypeError(EMPTY_ARR) : null;
-    done(err, initial);
+  if (len === 0 && !hasInitial) {
+    done(new TypeError(REDUCE_EMPTY_ARR), initial);
+    return;
+  }
+
+  let previous = hasInitial ? initial : items[0];
+  if ((len === 0 && hasInitial) || (len === 1 && !hasInitial)) {
+    done(null, previous);
     return;
   }
 
-  let previous = count === 1 ? items[0] : initial;
+  let count = hasInitial ? 0 : 1;
   let current = items[count];
   const last = len - 1;
 
diff --git a/test/array.reduce.js b/test/array.reduce.js
@@ -3,7 +3,7 @@
 const metasync = require('..');
 const metatests = require('metatests');
 
-metatests.test('successful with initial', test => {
+metatests.test('reduce with initial', test => {
   const arr = [1, 2, 3, 4, 5];
   const initial = 10;
   const expectedRes = 25;
@@ -54,7 +54,21 @@ metatests.test('reduce without initial and with empty array', test => {
   );
 });
 
-metatests.test('successful without initial', test => {
+metatests.test('reduce without initial and with single-element array', test => {
+  const arr = [2];
+
+  metasync.reduce(
+    arr,
+    (prev, cur, callback) => process.nextTick(() => callback(null, prev + cur)),
+    (err, res) => {
+      test.strictSame(err, null);
+      test.strictSame(res, 2);
+      test.end();
+    }
+  );
+});
+
+metatests.test('reduce without initial', test => {
   const arr = [1, 2, 3, 4, 5];
   const expectedRes = 15;
 
@@ -69,7 +83,7 @@ metatests.test('successful without initial', test => {
   );
 });
 
-metatests.test('successful with asymetric function', test => {
+metatests.test('reduce with asymetric function', test => {
   const arr = '10110011';
   const expectedRes = 179;
 
@@ -85,7 +99,7 @@ metatests.test('successful with asymetric function', test => {
   );
 });
 
-metatests.test('with error', test => {
+metatests.test('reduce with error', test => {
   const arr = '10120011';
   const reduceError = new Error('Reduce error');
 
