Merge pull request jashkenas#1689 from akre54/partial-placeholder

add support for changing _.partial's placeholder

test/functions.js

@@ -77,6 +77,16 @@
     ok(widget instanceof MyWidget, 'Can partially bind a constructor');
     equal(widget.get(), 'foo', 'keeps prototype');
     deepEqual(widget.options, {a: 1});
+
+    _.partial.placeholder = obj;
+    func = _.partial(function() { return arguments.length; }, obj, 'b', obj, 'd');
+    equal(func('a'), 4, 'allows the placeholder to be swapped out');
+
+    _.partial.placeholder = {};
+    func = _.partial(function() { return arguments.length; }, obj, 'b', obj, 'd');
+    equal(func('a'), 5, 'swapping the placeholder preserves previously bound arguments');
+
+    _.partial.placeholder = _;
   });
 
   test('bindAll', function() {
@@ -575,7 +585,7 @@
       deepEqual(_.toArray(cb(1, 2, 3)), _.range(1, 4));
       deepEqual(_.toArray(cb(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)), _.range(1, 11));
     });
-    
+
   });
 
 }());

underscore.js

@@ -697,21 +697,25 @@
 
   // 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.
+  // as a placeholder by default, allowing any combination of arguments to be
+  // pre-filled. Set `_.partial.placeholder` for a custom placeholder argument.
   _.partial = function(func) {
-    var boundArgs = slice.call(arguments, 1);
+    var boundArgs = slice.call(arguments, 1),
+        placeholder = _.partial.placeholder;
     var bound = function() {
       var position = 0, length = boundArgs.length;
       var args = Array(length);
       for (var i = 0; i < length; i++) {
-        args[i] = boundArgs[i] === _ ? arguments[position++] : boundArgs[i];
+        args[i] = boundArgs[i] === placeholder ? arguments[position++] : boundArgs[i];
       }
       while (position < arguments.length) args.push(arguments[position++]);
       return executeBound(func, bound, this, this, args);
     };
     return bound;
   };
 
+  _.partial.placeholder = _;
+
   // Bind a number of an object's methods to that object. Remaining arguments
   // are the method names to be bound. Useful for ensuring that all callbacks
   // defined on an object belong to it.