cloning an object

Author: Sebastian Slomski

chapters/objects/cloning.textile

@@ -0,0 +1,62 @@
+---
+layout: recipe
+title: Cloning an object (deep copy)
+chapter: Objects
+---
+
+h2. Problem
+
+You want to clone an object with all its sub-objects
+
+h2. Solution
+
+{% highlight coffeescript %}
+clone = (obj) ->
+  if not obj? or typeof obj isnt 'object'
+    return obj
+
+  newInstance = new obj.constructor()
+
+  for key of obj
+    newInstance[key] = clone obj[key]
+
+  return newInstance
+
+x =
+  foo: 'bar'
+  bar: 'foo'
+
+y = clone(x)
+
+y.foo = 'test'
+
+console.log x.foo isnt y.foo, x.foo, y.foo
+# => true, bar, test
+{% endhighlight %}
+
+h2. Discussion
+
+The difference between copying an object through assignement and through this
+clone-function is how they handle references.
+The assignement only copies the object's reference, whereas the clone-function
+creates a complete new object by
+* creating a new object like the source object,
+* copying all attributes form the source object to the new object and
+* repeating these steps for all sub-objects by calling the clone-function
+  recursively.
+
+Example of an assignement copy:
+{% highlight coffeescript %}
+x =
+  foo: 'bar'
+  bar: 'foo'
+
+y = x
+
+y.foo = 'test'
+
+console.log x.foo isnt y.foo, x.foo, y.foo
+# => false, test, test
+{% endhighlight %}
+
+As you can see, when you change `y` after the copy, you also change `x`.