From 433331adac95cc681e18b07f561bc36bef52799f Mon Sep 17 00:00:00 2001
From: Graeme Yeates <yeatesgraeme@gmail.com>
Date: Fri, 28 Aug 2015 23:15:02 -0400
Subject: [PATCH] Add ES6 compliant lodash(...).next() iterator.

---
 lodash.js    | 35 +++++++++++++++++++++++
 test/test.js | 79 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 114 insertions(+)

diff --git a/lodash.js b/lodash.js
index cab1b297f..81407ebdd 100644
--- a/lodash.js
+++ b/lodash.js
@@ -1492,6 +1492,8 @@
       this.__wrapped__ = value;
       this.__actions__ = actions || [];
       this.__chain__ = !!chainAll;
+      this.__index__ = 0;
+      this.__values__ = undefined;
     }
 
     /**
@@ -5867,6 +5869,38 @@
       });
     });
 
+    /**
+     * Gets the next value on a wrapped object in ES2015 iterator
+     * format (<link>). Useful for `for..of` loops and using with
+     * some ES2015 features.
+     *
+     * @name next
+     * @memberOf _
+     * @category Chain
+     * @returns {*} Returns the next value.
+     * @example
+     *
+     * var wrapped = _([1, 2]);
+     *
+     * wrapped.next();
+     * // => { 'done': false, 'value': 1 }
+     *
+     * wrapped.next();
+     * // => { 'done': false, 'value': 2 }
+     *
+     * wrapped.next();
+     * // => { 'done': true, 'value': undefined }
+     */
+    function wrapperNext() {
+      if (this.__values__ === undefined) {
+        this.__values__ = values(this.value());
+      }
+      var done = this.__index__ >= this.__values__.length,
+          value = done ? undefined : this.__values__[this.__index__++];
+
+      return { 'done': done, 'value': value };
+    }
+
     /**
      * Creates a clone of the chained sequence planting `value` as the wrapped value.
      *
@@ -11958,6 +11992,7 @@
     lodash.prototype.chain = wrapperChain;
     lodash.prototype.commit = wrapperCommit;
     lodash.prototype.concat = wrapperConcat;
+    lodash.prototype.next = wrapperNext;
     lodash.prototype.plant = wrapperPlant;
     lodash.prototype.reverse = wrapperReverse;
     lodash.prototype.toString = wrapperToString;
diff --git a/test/test.js b/test/test.js
index ae8d32c48..f8407c926 100644
--- a/test/test.js
+++ b/test/test.js
@@ -16580,6 +16580,85 @@
 
   /*--------------------------------------------------------------------------*/
 
+  QUnit.module('lodash(...).next');
+
+  _.each([true, false], function(implict) {
+    function arrayFrom(iter) {
+      var item;
+      var result = [];
+      while (!(item = iter.next()).done) {
+        result.push(item.value);
+      }
+      return result;
+    }
+
+    function chain(obj) {
+      return implict ? _(obj) : _.chain(obj);
+    }
+
+    var chainType = ' in an ' + (implict ? 'implict' : 'explict') + ' chain';
+
+    test('should produce an ES6 compliant object' + chainType, 7, function() {
+      var array = [0, 1, 1, 2, 3],
+          chained = chain(array);
+
+      deepEqual(chained.next(), {done: false, value: 0});
+      deepEqual(chained.next(), {done: false, value: 1});
+      deepEqual(chained.next(), {done: false, value: 1});
+      deepEqual(chained.next(), {done: false, value: 2});
+      deepEqual(chained.next(), {done: false, value: 3});
+      deepEqual(chained.next(), {done: true, value: undefined});
+      deepEqual(chained.next(), {done: true, value: undefined});
+    });
+
+    test('should make a lodash instance act as an iterable' + chainType, 1, function() {
+      var array = [0, 1, 1, 2, 3, 5, 8, 13, 21],
+          chained = chain(array);
+
+      deepEqual(arrayFrom(chained), array);
+    });
+
+    test('should reset the iterator upon forking (adding an action to the chain)' + chainType, 5, function() {
+      var array = [0, 1, 1, 2, 3, 5, 8, 13, 21],
+          chained = chain(array);
+
+      deepEqual(arrayFrom(chained), array);
+      // before reset produces empty array as iterator is exhausted
+      deepEqual(arrayFrom(chained), []);
+
+      var newChain = chained.filter(_.constant(true));
+      deepEqual(arrayFrom(newChain), array);
+
+      // original chain is still exhausted
+      deepEqual(arrayFrom(chained), []);
+
+      newChain = chained.slice();
+      deepEqual(arrayFrom(newChain), array);
+    });
+
+    test('should work in a lazy sequence' + chainType, 3, function() {
+      if (true) {
+        var array = _.range(1, LARGE_ARRAY_SIZE + 1),
+            values = [],
+            predicate = function(value) { values.push(value); return value > 99; },
+            chained = chain(array);
+
+        deepEqual(arrayFrom(chained), array);
+
+        // resets index & supports lazy array
+        chained = chained.filter(predicate);
+        deepEqual(arrayFrom(chained), array.slice(99));
+        // Doesn't recompute everything each time
+        deepEqual(values, array);
+      }
+      else {
+        skipTest(3);
+      }
+    });
+  });
+
+  /*--------------------------------------------------------------------------*/
+
   QUnit.module('lodash(...).plant');
 
   (function() {