Fix commonCharacters.js - add createHash function and add comments wi…

…thin commonCharacters function

commonCharacters/commonCharacters.js

@@ -1,6 +1,6 @@
 /**
  * Write a function `f(a, b)` which takes two strings as arguments and returns a
- * string containing only the unique characters found in both strings, in the 
+ * string containing only the unique characters found in both strings, in the
  * order that they appeared in `a`. Remember to skip spaces and characters you
  * have already encountered!
  *
@@ -10,7 +10,6 @@
  * Extra credit: Extend your function to handle more than two input strings.
  */
 
-
 var commonCharacters = function () {
   var args = Array.prototype.slice.call(arguments);
   if (args.length < 2) throw new Error('Require at least 2 input strings');
@@ -22,9 +21,16 @@ var commonCharacters = function () {
    * Key step here: keep on reducing the unique set until you get the result across
    * all strings
   */
+
+  /*
+   * The reason createHash(first) is needed is because in order to maintain a
+   * unique set after each reduction, you'll need to pass an initial object with
+   * with the characters so far. If an empty object is passed, then of course there
+   * are no other characters that will match against the empty object.
+  */
   var result = args.reduce(function (acc, cur) {
     return createUniqueSet(acc, cur);
-  });
+  }, createHash(first));
 
   for (var i = 0; i < first.length; i += 1) {
     if (result[first[i]]) {
@@ -36,13 +42,23 @@ var commonCharacters = function () {
   return str;
 };
 
-var createUniqueSet = function (a, b) {
-  var hashB  = {};
+var createHash = function (string) {
+  var result = {};
+  for (var i = 0; i < string.length; i += 1) {
+    result[ string[i] ] = true;
+  }
+  return result;
+};
+
+var createUniqueSet = function (obj, string) {
   var result = {};
+  var len    = string.length;
   var i;
 
-  for (i = 0; i < b.length; i += 1) {
-    result[b[i]] = true;
+  for (i = 0; i < len; i += 1) {
+    if (obj[ string[i] ]) {
+      result[ string[i] ] = true;
+    }
   }
 
   return result;