Add annotated javascript of Polyfill pattern rewrite

Author: d8uv

chapters/metaprogramming/detecting-and-replacing-functions.md

@@ -30,3 +30,33 @@ However, we don't ever want to overwrite a prototype that we haven't created in
 What you need to do is only add the function if it doesn't already exist. That's where the existential assignment operator (`?=`) comes in. If we do `Array::filter ?= ...` instead, it will see if `Array::filter` already exists. If it does, then it will use the current version. If it doesn't, it will add yours.
 
 Finally, because the existential assignment operator--when compiled--creates a few variables, we clean up the code by wrapping it in an [Immediately-Invoked Function Expression (IIFE)](http://benalman.com/news/2010/11/immediately-invoked-function-expression/). This hides those internal-use-only variables from leaking outside. So, if the function we're writing already exists, it runs, does basically nothing, and exits, affecting absolutely none of your code. But, if the function we're writing *doesn't* exist, we send out only the function we're writing as a closure, so only the function you've made affects the code. The internal workings of `?=` are hidden either way.
+
+### Example
+
+Below, we've compiled and annotated the coffeescript written in the solution above
+
+{% highlight javascript %}
+// (function(){ ... })() is an IIFE, compiled in thanks to `do ->`
+(function() {
+
+  // This is from the `?=` operator, used to check if Array.prototype.filter (`Array::filter`) exists.
+  // If it does, we set it to itself, and return. If it doesn't, then we set it to the function, and return the function.
+  // The IIFE is only used to hide _base and _ref from the outside world.
+  var _base, _ref;
+  return (_ref = (_base = Array.prototype).filter) != null ? _ref : _base.filter = function(callback) {
+
+    // `element for element in this when callback element`
+    var element, _i, _len, _results;
+    _results = [];
+    for (_i = 0, _len = this.length; _i < _len; _i++) {
+      element = this[_i];
+      if (callback(element)) {
+        _results.push(element);
+      }
+    }
+    return _results;
+    
+  };
+// The end of the IIFE from `do ->`
+})();
+{% endhighlight %}