Merge pull request jashkenas#1700 from megawac/compose

Breaking: Optimization for compose

test/functions.js

@@ -478,6 +478,22 @@
 
     composed = _.compose(greet, exclaim);
     equal(composed('moe'), 'hi: moe!', 'in this case, the functions are also commutative');
+
+    // f(g(h(x, y, z)))
+    function h(x, y, z) {
+      equal(arguments.length, 3, 'First function called with multiple args');
+      return z * y;
+    };
+    function g(x) {
+      equal(arguments.length, 1, 'Composed function is called with 1 argument');
+      return x;
+    };
+    function f(x) {
+      equal(arguments.length, 1, 'Composed function is called with 1 argument');
+      return x * 2;
+    };
+    composed = _.compose(f, g, h);
+    equal(composed(1, 2, 3), 12);
   });
 
   test('after', function() {

underscore.js

@@ -804,13 +804,14 @@
   // Returns a function that is the composition of a list of functions, each
   // consuming the return value of the function that follows.
   _.compose = function() {
-    var funcs = arguments;
+    var funcs = arguments, length = funcs.length;
     return function() {
-      var args = arguments;
-      for (var i = funcs.length - 1; i >= 0; i--) {
-        args = [funcs[i].apply(this, args)];
+      var idx = length - 1,
+          result = funcs[idx].apply(this, arguments);
+      while (idx--) {
+        result = funcs[idx].call(this, result);
       }
-      return args[0];
+      return result;
     };
   };