refactor(done): add freezed done object

lib/done.js

@@ -0,0 +1,3 @@
+'use strict';
+
+module.exports = Object.freeze({ done: true });

lib/evenly.js

@@ -3,6 +3,7 @@
 const unspread = require('spread-args').unspread;
 
 const createIterator = require('./create-iterator');
+const done = require('./done');
 
 /**
  * Returns an Iterator, that traverses iterators evenly.
@@ -77,7 +78,7 @@ function evenly(iterables) {
     function next() {
         // Exit if all iterators are traversed.
         if (empties.size === count) {
-            return { done: true };
+            return done;
         }
 
         // Go to the next iterator.

lib/object-entries-iterator.js

@@ -1,5 +1,7 @@
 'use strict';
 
+const done = require('./done');
+
 /**
  * Creates an iterator whose elements are arrays corresponding to the enumerable property [key, value] pairs
  * found directly upon object.
@@ -40,7 +42,7 @@ module.exports = class ObjectEntriesIterator {
     }
     next() {
         if (this._index === this._length) {
-            return { done: true };
+            return done;
         }
 
         const key = this._keys[this._index++];

lib/series.js

@@ -3,6 +3,7 @@
 const unspread = require('spread-args').unspread;
 
 const createIterator = require('./create-iterator');
+const done = require('./done');
 
 /**
  * Returns an Iterator, that traverses iterators in series.
@@ -49,7 +50,7 @@ function series(iterables) {
             while (next.done) {
                 // If iterators are ended, then exit.
                 if (iterables.length === 0) {
-                    return { done: true };
+                    return done;
                 }
 
                 iter = createIterator(iterables.shift(), { strict: false });

lib/value-iterator.js

@@ -1,5 +1,7 @@
 'use strict';
 
+const done = require('./done');
+
 /**
  * Iterator with specified value.
  *
@@ -22,7 +24,7 @@ module.exports = class ValueIterator {
     }
     next() {
         if (this._done) {
-            return { done: true };
+            return done;
         }
 
         this._done = true;