replacing all isType tests that relied on string comparisons with versions that check for existence of known methods and properties. Less safe, but more than an order of magnitude faster.

underscore.js

@@ -428,6 +428,13 @@
     return _.extend({}, obj);
   };
 
+  // Invokes interceptor with the obj, and then returns obj.
+  // The primary purpose of this method is to "tap into" a method chain, in order to perform operations on intermediate results within the chain.
+  _.tap = function(obj, interceptor) {
+    interceptor(obj);
+    return obj;
+  };
+
   // Perform a deep comparison to check if two objects are equal.
   _.isEqual = function(a, b) {
     // Check object identity.
@@ -474,11 +481,41 @@
     return !!(obj && obj.nodeType == 1);
   };
 
+  // Is a given value an array?
+  _.isArray = function(obj) {
+    return obj && obj.concat && obj.unshift;
+  };
+
   // Is a given variable an arguments object?
   _.isArguments = function(obj) {
     return obj && _.isNumber(obj.length) && !_.isArray(obj) && !propertyIsEnumerable.call(obj, 'length');
   };
 
+  // Is a given value a function?
+  _.isFunction = function(obj) {
+    return obj && obj.constructor && obj.call && obj.apply;
+  };
+
+  // Is a given value a string?
+  _.isString = function(obj) {
+    return obj === '' || (obj && obj.charCodeAt && obj.substr);
+  };
+
+  // Is a given value a number?
+  _.isNumber = function(obj) {
+    return toString.call(obj) === '[object Number]';
+  };
+
+  // Is a given value a date?
+  _.isDate = function(obj) {
+    return obj && obj.getTimezoneOffset && obj.setUTCFullYear;
+  };
+
+  // Is the given value a regular expression?
+  _.isRegExp = function(obj) {
+    return obj && obj.test && obj.exec && (obj.ignoreCase || obj.ignoreCase === false);
+  };
+
   // Is the given value NaN -- this one is interesting. NaN != NaN, and
   // isNaN(undefined) == true, so we make sure it's a number first.
   _.isNaN = function(obj) {
@@ -495,23 +532,6 @@
     return typeof obj == 'undefined';
   };
 
-  // Invokes interceptor with the obj, and then returns obj. 
-  // The primary purpose of this method is to "tap into" a method chain, in order to perform operations on intermediate results within the chain.
-  _.tap = function(obj, interceptor) {
-    interceptor(obj);
-    return obj;
-  }
-
-  // Define the isArray, isDate, isFunction, isNumber, isRegExp, and isString
-  // functions based on their toString identifiers.
-  var types = ['Array', 'Date', 'Function', 'Number', 'RegExp', 'String'];
-  for (var i=0, l=types.length; i<l; i++) {
-    (function() {
-      var identifier = '[object ' + types[i] + ']';
-      _['is' + types[i]] = function(obj) { return toString.call(obj) == identifier; };
-    })();
-  }
-
   /* -------------------------- Utility Functions: -------------------------- */
 
   // Run Underscore.js in noConflict mode, returning the '_' variable to its