From 0653cfdf85d959d43683f9fb0dc54ba97f2e207b Mon Sep 17 00:00:00 2001
From: Justin Ridgewell <justin@ridgewell.name>
Date: Wed, 8 Jul 2015 22:39:12 -0400
Subject: [PATCH 1/2] Add isIterationCall helper

---
 underscore.js | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/underscore.js b/underscore.js
index bea6d0ac5..fc8aec8f8 100644
--- a/underscore.js
+++ b/underscore.js
@@ -136,15 +136,35 @@
     };
   };
 
+  // Helper to determine if index is within the bounds of an array.
+  var isIndex = function(index, length) {
+    return index >= 0 && index < length;
+  };
+
   // Helper for collection methods to determine whether a collection
-  // should be iterated as an array or as an object
+  // should be iterated as an array or as an object.
   // Related: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-tolength
   // Avoids a very nasty iOS 8 JIT bug on ARM-64. #2094
   var MAX_ARRAY_INDEX = Math.pow(2, 53) - 1;
   var getLength = property('length');
   var isArrayLike = function(collection) {
     var length = getLength(collection);
-    return typeof length == 'number' && length >= 0 && length <= MAX_ARRAY_INDEX;
+    return typeof length == 'number' && isIndex(length, MAX_ARRAY_INDEX);
+  };
+
+  // Helper to determine if the arguments to a function are from
+  // an iteration of `each`, `map`, and friends.
+  var isIterationCall = function(value, index, collection) {
+    if (!_.isObject(collection)) return false;
+    var contains = typeof index === 'number' ?
+      isArrayLike(collection) && isIndex(index, collection.length) :
+      index in collection;
+    if (contains) {
+      var other = collection[index];
+      return value === value ? value === other : other !== other;
+    }
+
+    return false;
   };
 
   // Collection Functions

From 6e9da263d21319cec67011d947c3d35ddd6b96cc Mon Sep 17 00:00:00 2001
From: Justin Ridgewell <justin@ridgewell.name>
Date: Wed, 8 Jul 2015 22:39:29 -0400
Subject: [PATCH 2/2] Use isIterationCall to determine iteration calls

---
 underscore.js | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/underscore.js b/underscore.js
index fc8aec8f8..3ad360b28 100644
--- a/underscore.js
+++ b/underscore.js
@@ -291,7 +291,9 @@
   // Aliased as `includes` and `include`.
   _.contains = _.includes = _.include = function(obj, item, fromIndex, guard) {
     if (!isArrayLike(obj)) obj = _.values(obj);
-    if (typeof fromIndex != 'number' || guard) fromIndex = 0;
+    if (typeof fromIndex != 'number' || (guard && isIterationCall(item, fromIndex, guard))) {
+      fromIndex = 0;
+    }
     return _.indexOf(obj, item, fromIndex) >= 0;
   };
 
@@ -325,7 +327,8 @@
   _.max = function(obj, iteratee, context) {
     var result = -Infinity, lastComputed = -Infinity,
         value, computed;
-    if (iteratee == null || (typeof iteratee == 'number' && typeof obj[0] != 'object') && obj != null) {
+    if (context && isIterationCall(obj, iteratee, context)) iteratee = null;
+    if (iteratee == null) {
       obj = isArrayLike(obj) ? obj : _.values(obj);
       for (var i = 0, length = obj.length; i < length; i++) {
         value = obj[i];
@@ -350,7 +353,8 @@
   _.min = function(obj, iteratee, context) {
     var result = Infinity, lastComputed = Infinity,
         value, computed;
-    if (iteratee == null || (typeof iteratee == 'number' && typeof obj[0] != 'object') && obj != null) {
+    if (context && isIterationCall(obj, iteratee, context)) iteratee = null;
+    if (iteratee == null) {
       obj = isArrayLike(obj) ? obj : _.values(obj);
       for (var i = 0, length = obj.length; i < length; i++) {
         value = obj[i];
@@ -381,7 +385,7 @@
   // If **n** is not specified, returns a single random element.
   // The internal `guard` argument allows it to work with `map`.
   _.sample = function(obj, n, guard) {
-    if (n == null || guard) {
+    if (n == null || (guard && isIterationCall(obj, n, guard))) {
       if (!isArrayLike(obj)) obj = _.values(obj);
       return obj[_.random(obj.length - 1)];
     }
@@ -478,7 +482,7 @@
   // allows it to work with `_.map`.
   _.first = _.head = _.take = function(array, n, guard) {
     if (array == null) return void 0;
-    if (n == null || guard) return array[0];
+    if (n == null || (guard && isIterationCall(array, n, guard))) return array[0];
     return _.initial(array, array.length - n);
   };
 
@@ -486,14 +490,15 @@
   // the arguments object. Passing **n** will return all the values in
   // the array, excluding the last N.
   _.initial = function(array, n, guard) {
-    return slice.call(array, 0, Math.max(0, array.length - (n == null || guard ? 1 : n)));
+    if (n == null || (guard && isIterationCall(array, n, guard))) n = 1;
+    return slice.call(array, 0, Math.max(0, array.length - n));
   };
 
   // Get the last element of an array. Passing **n** will return the last N
   // values in the array.
   _.last = function(array, n, guard) {
     if (array == null) return void 0;
-    if (n == null || guard) return array[array.length - 1];
+    if (n == null || (guard && isIterationCall(array, n, guard))) return array[array.length - 1];
     return _.rest(array, Math.max(0, array.length - n));
   };
 
@@ -501,7 +506,8 @@
   // Especially useful on the arguments object. Passing an **n** will return
   // the rest N values in the array.
   _.rest = _.tail = _.drop = function(array, n, guard) {
-    return slice.call(array, n == null || guard ? 1 : n);
+    if (n == null || (guard && isIterationCall(array, n, guard))) n = 1;
+    return slice.call(array, n);
   };
 
   // Trim out all falsy values from an array.