Fixes #474 -- add _.partial

test/functions.js

@@ -18,10 +18,10 @@ $(document).ready(function() {
     func = _.bind(func, this, 'hello');
     equal(func('moe'), 'hello: moe', 'the function was partially applied in advance');
 
-    var func = _.bind(func, this, 'curly');
+    func = _.bind(func, this, 'curly');
     equal(func(), 'hello: curly', 'the function was completely applied in advance');
 
-    var func = function(salutation, firstname, lastname) { return salutation + ': ' + firstname + ' ' + lastname; };
+    func = function(salutation, firstname, lastname) { return salutation + ': ' + firstname + ' ' + lastname; };
     func = _.bind(func, this, 'hello', 'moe', 'curly');
     equal(func(), 'hello: moe curly', 'the function was partially applied in advance and can accept multiple arguments');
 
@@ -37,6 +37,14 @@ $(document).ready(function() {
     equal(Boundf().hello, "moe curly", "When called without the new operator, it's OK to be bound to the context");
   });
 
+  test("partial", function() {
+    var obj = {name: 'moe'};
+    var func = function() { return this.name + ' ' + _.toArray(arguments).join(' '); };
+
+    obj.func = _.partial(func, 'a', 'b');
+    equal(obj.func('c', 'd'), 'moe a b c d', 'can partially apply');
+  });
+
   test("bindAll", function() {
     var curly = {name : 'curly'}, moe = {
       name    : 'moe',

underscore.js

@@ -578,9 +578,8 @@
   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.
+  // optionally). Delegates to **ECMAScript 5**'s native `Function.bind` if
+  // available.
   _.bind = function(func, context) {
     if (func.bind === nativeBind && nativeBind) return nativeBind.apply(func, slice.call(arguments, 1));
     var args = slice.call(arguments, 2);
@@ -589,6 +588,15 @@
     };
   };
 
+  // Partially apply a function by creating a version that has had some of its
+  // arguments pre-filled, without changing its dynamic `this` context.
+  _.partial = function(func) {
+    var args = slice.call(arguments, 1);
+    return function() {
+      return func.apply(this, args.concat(slice.call(arguments)));
+    };
+  };
+
   // 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) {