From abb00ac7f02079a9a63eca813c91ed34ea35fb09 Mon Sep 17 00:00:00 2001
From: Sebastian Wilzbach <sebi@wilzbach.me>
Date: Thu, 29 May 2014 19:46:46 +0200
Subject: [PATCH] added explanation for prototype class variables

---
 .../classes_and_objects/class-variables.md    | 49 ++++++++++++++++++-
 1 file changed, 48 insertions(+), 1 deletion(-)

diff --git a/chapters/classes_and_objects/class-variables.md b/chapters/classes_and_objects/class-variables.md
index dd8713f..0b28b4e 100644
--- a/chapters/classes_and_objects/class-variables.md
+++ b/chapters/classes_and_objects/class-variables.md
@@ -40,4 +40,51 @@ zoo.helpfulInfo()
 
 ## Discussion
 
-Coffeescript will store these values on the class itself rather than on the prototype it defines.  These are useful for defining variables on classes which can't be overrided by instance attribute variables.
+Coffeescript will store these values on the class itself rather than on the prototype it defines. 
+These are useful for defining variables on classes which can not be overrided by instance attribute variables.
+
+## Warning
+
+Be aware that for non-primitive (mutable) properties (anything execpt `number`, `string`, `boolean`, `null` or `undefined`)
+so (e.g. an array `[]` or a dictionary `{}`)  the variable will be shared among instances.
+
+## Problem
+
+You want to create a class variable for non-primitive type.
+
+## Solution
+
+Create the variable in the constructor
+
+
+{% highlight coffeescript %}
+class Animal 
+
+	# variable is mutable and will be shared across all instances of the prototype
+	favoriteFoodShared : []
+	
+	constructor: ->
+		@favoriteFood = []
+
+bear = new Animal()
+lion = new Animal()
+
+lion.favoriteFood.push "Zebras"
+
+lion.favoriteFood
+# => ["Zebras"]
+
+bear.favoriteFood
+# => []
+
+# here is what happens if you would use modify the mutable variable
+
+lion.favoriteFoodShared.push "Zebras"
+
+lion.favoriteFoodShared
+# => ["Zebras"]
+
+bear.favoriteFoodShared
+# => ["Zebras"]
+
+{% endhighlight %}