Much better strategy for calling native Array methods.

Compare an object's function with === to the native
version before calling it.  That is, don't just assume that
because an object has a 'filter' property thats a function,
that we should call that in _.filter().

Expose the native methods found as _.native

On the way towards being able to turn off native implementations,
though there are tradeoffs...

underscore.js

@@ -37,6 +37,25 @@
       hasOwnProperty        = Object.prototype.hasOwnProperty,
       propertyIsEnumerable  = Object.prototype.propertyIsEnumerable;
 
+  // All native implementations we hope to use are declared here, 
+  // mostly Array.prototype methods.
+  //
+  // 'native' is on the long list of reserved words.  
+  // Ok to use as a property name, though jsLint doesn't agree.
+  var Native = _['native'] = {
+    forEach:        Array.prototype.forEach,
+    map:            Array.prototype.map,
+    reduce:         Array.prototype.reduce,
+    reduceRight:    Array.prototype.reduceRight,
+    filter:         Array.prototype.filter,
+    every:          Array.prototype.every,
+    some:           Array.prototype.some,
+    indexOf:        Array.prototype.indexOf,
+    lastIndexOf:    Array.prototype.lastIndexOf,
+    isArray:        Array.isArray,
+    keys:           Object.keys
+  };
+  
   // Current version.
   _.VERSION = '0.5.8';
 
@@ -49,7 +68,7 @@
   _.forEach = function(obj, iterator, context) {
     var index = 0;
     try {
-      if (obj.forEach) {
+      if (obj.forEach === Native.forEach) {
         obj.forEach(iterator, context);
       } else if (_.isNumber(obj.length)) {
         for (var i=0, l=obj.length; i<l; i++) iterator.call(context, obj[i], i, obj);
@@ -66,7 +85,7 @@
   // Return the results of applying the iterator to each element. 
   // Delegates to JavaScript 1.6's native map if available.
   _.map = function(obj, iterator, context) {
-    if (obj && _.isFunction(obj.map)) return obj.map(iterator, context);
+    if (obj.map === Native.map) return obj.map(iterator, context);
     var results = [];
     each(obj, function(value, index, list) {
       results.push(iterator.call(context, value, index, list));
@@ -78,7 +97,7 @@
   // inject, or foldl.
   // Delegates to JavaScript 1.8's native reduce if available.
   _.reduce = function(obj, memo, iterator, context) {
-    if (obj && _.isFunction(obj.reduce)) return obj.reduce(_.bind(iterator, context), memo);
+    if (obj.reduce === Native.reduce) return obj.reduce(_.bind(iterator, context), memo);
     each(obj, function(value, index, list) {
       memo = iterator.call(context, memo, value, index, list);
     });
@@ -88,7 +107,7 @@
   // The right-associative version of reduce, also known as foldr. Uses
   // Delegates to JavaScript 1.8's native reduceRight if available.
   _.reduceRight = function(obj, memo, iterator, context) {
-    if (obj && _.isFunction(obj.reduceRight)) return obj.reduceRight(_.bind(iterator, context), memo);
+    if (obj.reduceRight === Native.reduceRight) return obj.reduceRight(_.bind(iterator, context), memo);
     var reversed = _.clone(_.toArray(obj)).reverse();
     return reduce(reversed, memo, iterator, context);
   };
@@ -108,7 +127,7 @@
   // Return all the elements that pass a truth test. 
   // Delegates to JavaScript 1.6's native filter if available.
   _.filter = function(obj, iterator, context) {
-    if (obj && _.isFunction(obj.filter)) return obj.filter(iterator, context);
+    if (obj.filter === Native.filter) return obj.filter(iterator, context);
     var results = [];
     each(obj, function(value, index, list) {
       iterator.call(context, value, index, list) && results.push(value);
@@ -129,7 +148,7 @@
   // Delegates to JavaScript 1.6's native every if available.
   _.every = function(obj, iterator, context) {
     iterator = iterator || _.identity;
-    if (obj && _.isFunction(obj.every)) return obj.every(iterator, context);
+    if (obj.every === Native.every) return obj.every(iterator, context);
     var result = true;
     each(obj, function(value, index, list) {
       if (!(result = result && iterator.call(context, value, index, list))) _.breakLoop();
@@ -141,7 +160,7 @@
   // Delegates to JavaScript 1.6's native some if available.
   _.some = function(obj, iterator, context) {
     iterator = iterator || _.identity;
-    if (obj && _.isFunction(obj.some)) return obj.some(iterator, context);
+    if (obj.some === Native.some) return obj.some(iterator, context);
     var result = false;
     each(obj, function(value, index, list) {
       if (result = iterator.call(context, value, index, list)) _.breakLoop();
@@ -326,7 +345,7 @@
   // item in an array, or -1 if the item is not included in the array.
   // Delegates to JavaScript 1.8's native indexOf if available.
   _.indexOf = function(array, item) {
-    if (array.indexOf) return array.indexOf(item);
+    if (array.indexOf === Native.indexOf) return array.indexOf(item);
     for (var i=0, l=array.length; i<l; i++) if (array[i] === item) return i;
     return -1;
   };
@@ -334,7 +353,7 @@
 
   // Delegates to JavaScript 1.6's native lastIndexOf if available.
   _.lastIndexOf = function(array, item) {
-    if (array.lastIndexOf) return array.lastIndexOf(item);
+    if (array.lastIndexOf === Native.lastIndexOf) return array.lastIndexOf(item);
     var i = array.length;
     while (i--) if (array[i] === item) return i;
     return -1;
@@ -416,7 +435,7 @@
 
   // Retrieve the names of an object's properties.
   // Delegates to ECMA5's native Object.keys
-  _.keys = Object.keys || function(obj) {
+  _.keys = Native.keys || function(obj) {
     if (_.isArray(obj)) return _.range(0, obj.length);
     var keys = [];
     for (var key in obj) if (hasOwnProperty.call(obj, key)) keys.push(key);
@@ -515,7 +534,7 @@
 
   // Is a given array or object empty?
   _.isEmpty = function(obj) {
-    if (_.isArray(obj)) return obj.length===0;
+    if (_.isArray(obj)) return obj.length === 0;
     for (var k in obj) return false;
     return true;
   };
@@ -527,7 +546,7 @@
 
   // Is a given value an array?
   // Delegates to ECMA5's native Array.isArray
-  _.isArray = Array.isArray || function(obj) {
+  _.isArray = Native.isArray || function(obj) {
     return !!(obj && obj.concat && obj.unshift);
   };