Enhancing _.bind() to support binding constructors ... which you would never, ever want to do.

test/functions.js

@@ -29,6 +29,13 @@ $(document).ready(function() {
     _.bind(func, 0, 0, 'can bind a function to `0`')();
     _.bind(func, '', '', 'can bind a function to an empty string')();
     _.bind(func, false, false, 'can bind a function to `false`')();
+
+    // These tests are only meaningful when using a browser without a native bind function
+    // To test this with a modern browser, set underscore's nativeBind to undefined
+    var F = function () { return this; };
+    var Boundf = _.bind(F, {hello: "moe curly"});
+    equal(new Boundf().hello, undefined, "function should not be bound to the context, to comply with ECMAScript 5");
+    equal(Boundf().hello, "moe curly", "When called without the new operator, it's OK to be bound to the context");
   });
 
   test("functions: bindAll", function() {

underscore.js

@@ -464,15 +464,25 @@
   // Function (ahem) Functions
   // ------------------
 
+  // Reusable constructor function for prototype setting.
+  var ctor = function(){};
+
   // Create a function bound to a given object (assigning `this`, and arguments,
   // optionally). Binding with arguments is also known as `curry`.
   // Delegates to **ECMAScript 5**'s native `Function.bind` if available.
   // We check for `func.bind` first, to fail fast when `func` is undefined.
-  _.bind = function(func, obj) {
+  _.bind = function bind(func, context) {
+    var bound, args;
     if (func.bind === nativeBind && nativeBind) return nativeBind.apply(func, slice.call(arguments, 1));
-    var args = slice.call(arguments, 2);
-    return function() {
-      return func.apply(obj, args.concat(slice.call(arguments)));
+    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)));
+      ctor.prototype = func.prototype;
+      var self = new ctor;
+      var result = func.apply(self, args.concat(slice.call(arguments)));
+      if (Object(result) === result) return result;
+      return self;
     };
   };