Partial support for constructors

test/functions.js

@@ -59,6 +59,20 @@
 
     func = _.partial(function() { return typeof arguments[2]; }, _, 'b', _, 'd');
     equal(func('a'), 'undefined', 'unfilled placeholders are undefined');
+
+    // passes context
+    function MyWidget(name, options) {
+      this.name = name;
+      this.options = options;
+    }
+    MyWidget.prototype.get = function() {
+      return this.name;
+    };
+    var MyWidgetWithCoolOpts = _.partial(MyWidget, _, {a: 1});
+    var widget = new MyWidgetWithCoolOpts('foo');
+    ok(widget instanceof MyWidget, 'Can partially bind a constructor');
+    equal(widget.get(), 'foo', 'keeps prototype');
+    deepEqual(widget.options, {a: 1});
   });
 
   test('bindAll', function() {

underscore.js

@@ -645,39 +645,43 @@
   // Reusable constructor function for prototype setting.
   var Ctor = function(){};
 
+  // Determines whether to execute a function as a constructor
+  // or a normal function with the provided arguments
+  var executeBound = function(sourceFunc, boundFunc, context, callingContext, args) {
+    if (!(callingContext instanceof boundFunc)) return sourceFunc.apply(context, args);
+    Ctor.prototype = sourceFunc.prototype;
+    var self = new Ctor;
+    Ctor.prototype = null;
+    var result = sourceFunc.apply(self, args);
+    if (_.isObject(result)) return result;
+    return self;
+  };
+
   // Create a function bound to a given object (assigning `this`, and arguments,
   // optionally). Delegates to **ECMAScript 5**'s native `Function.bind` if
   // available.
   _.bind = function(func, context) {
-    var args, bound;
     if (nativeBind && func.bind === nativeBind) return nativeBind.apply(func, slice.call(arguments, 1));
-    if (!_.isFunction(func)) throw new TypeError('Bind must be called on a function');
-    args = slice.call(arguments, 2);
-    bound = function() {
-      if (!(this instanceof bound)) return func.apply(context, args.concat(slice.call(arguments)));
-      Ctor.prototype = func.prototype;
-      var self = new Ctor;
-      Ctor.prototype = null;
-      var result = func.apply(self, args.concat(slice.call(arguments)));
-      if (_.isObject(result)) return result;
-      return self;
+    if (!_.isFunction(func)) throw TypeError('Bind must be called on a function');
+    var args = slice.call(arguments, 2);
+    return function bound() {
+      return executeBound(func, bound, context, this, args.concat(slice.call(arguments)));
     };
-    return bound;
   };
 
   // Partially apply a function by creating a version that has had some of its
   // arguments pre-filled, without changing its dynamic `this` context. _ acts
   // as a placeholder, allowing any combination of arguments to be pre-filled.
   _.partial = function(func) {
     var boundArgs = slice.call(arguments, 1);
-    return function() {
+    return function bound() {
       var position = 0;
       var args = boundArgs.slice();
       for (var i = 0, length = args.length; i < length; i++) {
         if (args[i] === _) args[i] = arguments[position++];
       }
       while (position < arguments.length) args.push(arguments[position++]);
-      return func.apply(this, args);
+      return executeBound(func, bound, this, this, args);
     };
   };