diff --git a/underscore.js b/underscore.js
index 86e262343..5e1474c3e 100644
--- a/underscore.js
+++ b/underscore.js
@@ -24,7 +24,8 @@
   var ArrayProto = Array.prototype, ObjProto = Object.prototype, FuncProto = Function.prototype;
 
   // Create quick reference variables for speed access to core prototypes.
-  var slice            = ArrayProto.slice,
+  var concat           = ArrayProto.concat,
+      slice            = ArrayProto.slice,
       unshift          = ArrayProto.unshift,
       toString         = ObjProto.toString,
       hasOwnProperty   = ObjProto.hasOwnProperty;
@@ -72,6 +73,38 @@
   // Collection Functions
   // --------------------
 
+  // list of possible shadowed properties of Object.prototype
+  var shadowed = [
+    'constructor', 'hasOwnProperty', 'isPrototypeOf',
+    'propertyIsEnumerable', 'toLocaleString',
+    'toString', 'valueOf'
+  ];
+
+  // IE < 9 skips enumerable properties shadowing non-enumerable ones
+  var forShadowed = !{valueOf:0}.propertyIsEnumerable('valueOf') &&
+    function(obj, iterator) {
+      // because IE < 9 can't set the [[Enumerable]] attribute of an existing
+      // property and the `constructor` property of a prototype defaults to
+      // non-enumerable, we manually skip the `constructor` property when we
+      // think we are iterating over a `prototype` object.
+      var ctor = obj.constructor;
+      var skipCtor = ctor && ctor.prototype && ctor.prototype.constructor == ctor;
+      for (var key, i = 0; key = shadowed[i]; i++) {
+        if (!(skipCtor && key == 'constructor') &&
+            hasOwnProperty.call(obj, key) &&
+            iterator(obj[key], key, obj) === breaker) {
+          break;
+        }
+      }
+    };
+
+  var simpleEach = function(obj, iterator, index) {
+    index || (index = 0);
+    for(var l = obj.length; index < l; index++) {
+      iterator(obj[index]);
+    }
+  };
+
   // The cornerstone, an `each` implementation, aka `forEach`.
   // Handles objects with the built-in `forEach`, arrays, and raw objects.
   // Delegates to **ECMAScript 5**'s native `forEach` if available.
@@ -79,16 +112,42 @@
     if (obj == null) return;
     if (nativeForEach && obj.forEach === nativeForEach) {
       obj.forEach(iterator, context);
-    } else if (obj.length === +obj.length) {
-      for (var i = 0, l = obj.length; i < l; i++) {
-        if (i in obj && iterator.call(context, obj[i], i, obj) === breaker) return;
-      }
     } else {
-      for (var key in obj) {
-        if (hasOwnProperty.call(obj, key)) {
-          if (iterator.call(context, obj[key], key, obj) === breaker) return;
-        }
+      var fn = iterator;
+      var i = -1;
+      var l = obj.length;
+      if (context) {
+        iterator = function() { return fn.call(context, obj[i], i, obj); };
       }
+      if (l === l >>> 0) {
+        while (++i < l) {
+          if (i in obj && iterator(obj[i], i, obj) == breaker) return;
+        }
+      } else {
+        forProps(obj, iterator, true);
+      }
+    }
+  };
+
+  // Iterates over an object's properties, executing the `callback` for each.
+  var forProps = function(obj, iterator, ownOnly, context) {
+    var done = !obj;
+    var skipProto = typeof obj == 'function';
+
+    for (var key in obj) {
+      // Opera < 12 and Safari < 5.1 (if the prototype or a property on the prototype has been set)
+      // incorrectly set a function's `prototype` property [[Enumerable]] value
+      // to true. Because of this we standardize on skipping the the `prototype`
+      // property of functions regardless of their [[Enumerable]] value.
+      if (done =
+          !(skipProto && key == 'prototype') &&
+          (!ownOnly || ownOnly && hasOwnProperty.call(obj, key)) &&
+          iterator.call(context, obj[key], key, obj) === breaker) {
+        break;
+      }
+    }
+    if (!done && forShadowed) {
+      forShadowed(obj, iterator, context);
     }
   };
 
@@ -189,7 +248,7 @@
   // Delegates to **ECMAScript 5**'s native `some` if available.
   // Aliased as `any`.
   var any = _.some = _.any = function(obj, iterator, context) {
-    iterator = iterator || _.identity;
+    iterator || (iterator = _.identity);
     var result = false;
     if (obj == null) return result;
     if (nativeSome && obj.some === nativeSome) return obj.some(iterator, context);
@@ -228,7 +287,7 @@
   _.max = function(obj, iterator, context) {
     if (!iterator && _.isArray(obj)) return Math.max.apply(Math, obj);
     if (!iterator && _.isEmpty(obj)) return -Infinity;
-    var result = {computed : -Infinity};
+    var result = { computed : -Infinity };
     each(obj, function(value, index, list) {
       var computed = iterator ? iterator.call(context, value, index, list) : value;
       computed >= result.computed && (result = {value : value, computed : computed});
@@ -240,7 +299,7 @@
   _.min = function(obj, iterator, context) {
     if (!iterator && _.isArray(obj)) return Math.min.apply(Math, obj);
     if (!iterator && _.isEmpty(obj)) return Infinity;
-    var result = {computed : Infinity};
+    var result = { computed : Infinity };
     each(obj, function(value, index, list) {
       var computed = iterator ? iterator.call(context, value, index, list) : value;
       computed < result.computed && (result = {value : value, computed : computed});
@@ -267,11 +326,12 @@
   _.sortBy = function(obj, iterator, context) {
     return _.pluck(_.map(obj, function(value, index, list) {
       return {
-        value : value,
-        criteria : iterator.call(context, value, index, list)
+        value: value,
+        criteria: iterator.call(context, value, index, list)
       };
     }).sort(function(left, right) {
-      var a = left.criteria, b = right.criteria;
+      var a = left.criteria;
+      var b = right.criteria;
       return a < b ? -1 : a > b ? 1 : 0;
     }), 'value');
   };
@@ -292,7 +352,8 @@
   // be inserted so as to maintain order. Uses binary search.
   _.sortedIndex = function(array, obj, iterator) {
     iterator || (iterator = _.identity);
-    var low = 0, high = array.length;
+    var low = 0;
+    var high = array.length;
     while (low < high) {
       var mid = (low + high) >> 1;
       iterator(array[mid]) < iterator(obj) ? low = mid + 1 : high = mid;
@@ -302,10 +363,9 @@
 
   // Safely convert anything iterable into a real, live array.
   _.toArray = function(iterable) {
-    if (!iterable)                return [];
-    if (iterable.toArray)         return iterable.toArray();
-    if (_.isArray(iterable))      return slice.call(iterable);
-    if (_.isArguments(iterable))  return slice.call(iterable);
+    if (!iterable) return [];
+    if (iterable.toArray) return iterable.toArray();
+    if (_.isArray(iterable) || _.isArguments(iterable)) return slice.call(iterable);
     return _.values(iterable);
   };
 
@@ -420,10 +480,9 @@
   // Zip together multiple lists into a single array -- elements that share
   // an index go together.
   _.zip = function() {
-    var args = slice.call(arguments);
-    var length = _.max(_.pluck(args, 'length'));
+    var length = _.max(_.pluck(arguments, 'length'));
     var results = new Array(length);
-    for (var i = 0; i < length; i++) results[i] = _.pluck(args, "" + i);
+    for (var i = 0; i < length; i++) results[i] = _.pluck(arguments, i);
     return results;
   };
 
@@ -472,7 +531,6 @@
       range[idx++] = start;
       start += step;
     }
-
     return range;
   };
 
@@ -492,10 +550,10 @@
     if (!_.isFunction(func)) throw new TypeError;
     args = slice.call(arguments, 2);
     return bound = function() {
-      if (!(this instanceof bound)) return func.apply(context, args.concat(slice.call(arguments)));
+      if (!(this instanceof bound)) return func.apply(context, concat.apply(args, arguments));
       ctor.prototype = func.prototype;
       var self = new ctor;
-      var result = func.apply(self, args.concat(slice.call(arguments)));
+      var result = func.apply(self, concat.apply(args, arguments));
       if (Object(result) === result) return result;
       return self;
     };
@@ -504,16 +562,20 @@
   // Bind all of an object's methods to that object. Useful for ensuring that
   // all callbacks defined on an object belong to it.
   _.bindAll = function(obj) {
-    var funcs = slice.call(arguments, 1);
-    if (funcs.length == 0) funcs = _.functions(obj);
-    each(funcs, function(f) { obj[f] = _.bind(obj[f], obj); });
+    var i = 1;
+    var funcs = arguments;
+    if (funcs.length < 2) {
+      i = 0;
+      funcs = _.functions(obj);
+    }
+    simpleEach(funcs, function(f) { obj[f] = _.bind(obj[f], obj); }, i);
     return obj;
   };
 
   // Memoize an expensive function by storing its results.
   _.memoize = function(func, hasher) {
-    var memo = {};
     hasher || (hasher = _.identity);
+    var memo = {};
     return function() {
       var key = hasher.apply(this, arguments);
       return hasOwnProperty.call(memo, key) ? memo[key] : (memo[key] = func.apply(this, arguments));
@@ -588,7 +650,7 @@
   // conditionally execute the original function.
   _.wrap = function(func, wrapper) {
     return function() {
-      var args = [func].concat(slice.call(arguments));
+      var args = concat.apply([func], arguments);
       return wrapper.apply(this, args);
     };
   };
@@ -596,9 +658,9 @@
   // Returns a function that is the composition of a list of functions, each
   // consuming the return value of the function that follows.
   _.compose = function() {
-    var funcs = slice.call(arguments);
+    var funcs = arguments;
     return function() {
-      var args = slice.call(arguments);
+      var args = arguments;
       for (var i = funcs.length - 1; i >= 0; i--) {
         args = [funcs[i].apply(this, args)];
       }
@@ -622,7 +684,9 @@
   _.keys = nativeKeys || function(obj) {
     if (obj !== Object(obj)) throw new TypeError('Invalid object');
     var keys = [];
-    for (var key in obj) if (hasOwnProperty.call(obj, key)) keys[keys.length] = key;
+    forProps(obj, function(value, key) {
+      keys[keys.length] = key;
+    }, true);
     return keys;
   };
 
@@ -635,29 +699,29 @@
   // Aliased as `methods`
   _.functions = _.methods = function(obj) {
     var names = [];
-    for (var key in obj) {
-      if (_.isFunction(obj[key])) names.push(key);
-    }
+    forProps(obj, function(value, key) {
+      if (_.isFunction(value)) names[names.length] = key;
+    });
     return names.sort();
   };
 
   // Extend a given object with all the properties in passed-in object(s).
   _.extend = function(obj) {
-    each(slice.call(arguments, 1), function(source) {
-      for (var prop in source) {
-        if (source[prop] !== void 0) obj[prop] = source[prop];
-      }
-    });
+    simpleEach(arguments, function(source) {
+      forProps(source, function(value, key) {
+        if (value !== void 0) obj[key] = value;
+      });
+    }, 1);
     return obj;
   };
 
   // Fill in a given object with default properties.
   _.defaults = function(obj) {
-    each(slice.call(arguments, 1), function(source) {
-      for (var prop in source) {
-        if (obj[prop] == null) obj[prop] = source[prop];
-      }
-    });
+    simpleEach(arguments, function(source) {
+      forProps(source, function(value, key) {
+        if (obj[key] == null) obj[key] = value;
+      });
+    }, 1);
     return obj;
   };
 
@@ -686,8 +750,8 @@
     if (a._chain) a = a._wrapped;
     if (b._chain) b = b._wrapped;
     // Invoke a custom `isEqual` method if one is provided.
-    if (_.isFunction(a.isEqual)) return a.isEqual(b);
-    if (_.isFunction(b.isEqual)) return b.isEqual(a);
+    if (a.isEqual && _.isFunction(a.isEqual)) return a.isEqual(b);
+    if (b.isEqual && _.isFunction(b.isEqual)) return b.isEqual(a);
     // Compare `[[Class]]` names.
     var className = toString.call(a);
     if (className != toString.call(b)) return false;
@@ -696,13 +760,11 @@
       case '[object String]':
         // Primitives and their corresponding object wrappers are equivalent; thus, `"5"` is
         // equivalent to `new String("5")`.
-        return String(a) == String(b);
+        return a == String(b);
       case '[object Number]':
-        a = +a;
-        b = +b;
         // `NaN`s are equivalent, but non-reflexive. An `egal` comparison is performed for
         // other numeric values.
-        return a != a ? b != b : (a == 0 ? 1 / a == 1 / b : a == b);
+        return a != +a ? b != +b : (a == 0 ? 1 / a == 1 / b : a == +b);
       case '[object Date]':
       case '[object Boolean]':
         // Coerce dates and booleans to numeric primitive values. Dates are compared by their
@@ -742,21 +804,19 @@
       }
     } else {
       // Objects with different constructors are not equivalent.
-      if ("constructor" in a != "constructor" in b || a.constructor != b.constructor) return false;
+      if ('constructor' in a != 'constructor' in b || a.constructor != b.constructor) return false;
       // Deep compare objects.
-      for (var key in a) {
-        if (hasOwnProperty.call(a, key)) {
-          // Count the expected number of properties.
-          size++;
-          // Deep compare each member.
-          if (!(result = hasOwnProperty.call(b, key) && eq(a[key], b[key], stack))) break;
-        }
-      }
+      forProps(a, function(value, key) {
+        // Count the expected number of properties.
+        size++;
+        // Deep compare each member.
+        if (!(result = hasOwnProperty.call(b, key) && eq(value, b[key], stack))) return breaker;
+      }, true);
       // Ensure that both objects contain the same number of properties.
       if (result) {
-        for (key in b) {
-          if (hasOwnProperty.call(b, key) && !(size--)) break;
-        }
+        forProps(b, function() {
+          return !(size--) && breaker;
+        }, true);
         result = !size;
       }
     }
@@ -773,9 +833,15 @@
   // Is a given array, string, or object empty?
   // An "empty" object has no enumerable own-properties.
   _.isEmpty = function(obj) {
-    if (_.isArray(obj) || _.isString(obj)) return obj.length === 0;
-    for (var key in obj) if (hasOwnProperty.call(obj, key)) return false;
-    return true;
+    var result = toString.call(obj);
+    if (result == '[object Array]' || result == '[object String]') {
+      return !obj.length;
+    }
+    forProps(obj, function() {
+      result = false;
+      return breaker;
+    }, true);
+    return !!result;
   };
 
   // Is a given value a DOM element?
@@ -795,11 +861,11 @@
   };
 
   // Is a given variable an arguments object?
-  if (toString.call(arguments) == '[object Arguments]') {
-    _.isArguments = function(obj) {
-      return toString.call(obj) == '[object Arguments]';
-    };
-  } else {
+  _.isArguments = function(obj) {
+    return toString.call(obj) == '[object Arguments]';
+  };
+
+  if (!_.isArguments(arguments)) {
     _.isArguments = function(obj) {
       return !!(obj && hasOwnProperty.call(obj, 'callee'));
     };
@@ -879,7 +945,7 @@
   // Add your own custom functions to the Underscore object, ensuring that
   // they're correctly added to the OOP wrapper as well.
   _.mixin = function(obj) {
-    each(_.functions(obj), function(name){
+    simpleEach(_.functions(obj), function(name){
       addToWrapper(name, _[name] = obj[name]);
     });
   };
@@ -949,6 +1015,9 @@
   // A method to easily add functions to the OOP wrapper.
   var addToWrapper = function(name, func) {
     wrapper.prototype[name] = function() {
+      // avoid applying unshift generically to the arguments object because
+      // Opera < 9.50 will pave the value at index 0 without incrimenting the
+      // other index/values
       var args = slice.call(arguments);
       unshift.call(args, this._wrapped);
       return result(func.apply(_, args), this._chain);
@@ -959,7 +1028,7 @@
   _.mixin(_);
 
   // Add all mutator Array functions to the wrapper.
-  each(['pop', 'push', 'reverse', 'shift', 'sort', 'splice', 'unshift'], function(name) {
+  simpleEach(['pop', 'push', 'reverse', 'shift', 'sort', 'splice', 'unshift'], function(name) {
     var method = ArrayProto[name];
     wrapper.prototype[name] = function() {
       method.apply(this._wrapped, arguments);
@@ -968,7 +1037,7 @@
   });
 
   // Add all accessor Array functions to the wrapper.
-  each(['concat', 'join', 'slice'], function(name) {
+  simpleEach(['concat', 'join', 'slice'], function(name) {
     var method = ArrayProto[name];
     wrapper.prototype[name] = function() {
       return result(method.apply(this._wrapped, arguments), this._chain);