Adding _.count to count truthy values in an iterator. _.count([1, 2, 3, 4, 5, 6], function(num){ return num % 2 == 0; }) = 3

underscore.js

@@ -164,6 +164,16 @@
     return results;
   };
 
+  // Returns a count of elements which pass a truth test.
+  _.count = function(obj, iterator, context) {
+    var count = 0;
+    iterator = iterator || _.identity;
+    each(obj, function(value, index, list) {
+      if (iterator.call(context, value, index, list)) count += 1;
+    });
+    return count;
+  };
+  
   // Determine whether all of the elements match a truth test.
   // Delegates to **ECMAScript 5**'s native `every` if available.
   // Aliased as `all`.