use call to bind the current array element to this

build/parts/each.js

@@ -1,7 +1,7 @@
 $.each = function(arr, callback) {
   var i = 0, l = arr.length;
   for(; i < l; ++i) {
-    callback(i, arr[i]);
+    callback.call(arr[i], i, arr[i]);
   }
   return this;
 };

ki.extend.js

@@ -12,7 +12,7 @@
 $.each = function(arr, callback) {
   var i = 0, l = arr.length;
   for(; i < l; ++i) {
-    callback(i, arr[i]);
+    callback.call(arr[i], i, arr[i]);
   }
   return this;
 };

test/each-arrays/index.html

@@ -21,7 +21,7 @@
     document.addEventListener('DOMContentLoaded', function() {
       var arr = ['a','b','c'];
       var newarr = $.each(arr, function(index, elem){
-        log(index, elem);
+        log(index, elem, this);
       }).map(arr, function(item){
         return item + 'a';
       });