Restore only the essence of former optimizeCb

Author: jgonggrijp

modules/_bindCb.js

@@ -1,5 +1,5 @@
-// Internal function that returns a bound version of the passed-in callback, to
-// be repeatedly applied in other Underscore functions.
+// Internal function that returns a bound version of the
+// passed-in callback, used in `_.iteratee`.
 export default function bindCb(func, context) {
   if (context === void 0) return func;
   return function() {

modules/_bindCb4.js

@@ -0,0 +1,11 @@
+// In Firefox, `Function.prototype.call` is faster than
+// `Function.prototype.apply`. In the optimized variant of
+// `bindCb` below, we exploit the fact that no Underscore
+// function passes more than four arguments to a callback.
+// **NOT general enough for use outside of Underscore.**
+export default function bindCb4(func, context) {
+  if (context === void 0) return func;
+  return function(a1, a2, a3, a4) {
+    return func.call(context, a1, a2, a3, a4);
+  };
+}

modules/_cb.js

@@ -1,8 +1,12 @@
+import isFunction from './isFunction.js';
+import bindCb4 from './_bindCb4.js';
 import _ from './underscore.js';
 import './iteratee.js';
 
-// The function we call internally to generate a callback. It is just a
-// shorthand to save some bytes in the minified code.
+// The function we call internally to generate a callback: a wrapper
+// of `_.iteratee`, which uses `bindCb4` instead of `bindCb` for
+// function iteratees. It also saves some bytes in the minified code.
 export default function cb(value, context) {
+  if (isFunction(value)) return bindCb4(value, context);
   return _.iteratee(value, context);
 }