Cloning an Object (Deep Copy)

Problem

You want to clone an object with all its sub-objects.

Solution

clone = (obj) ->
  if not obj? or typeof obj isnt 'object'
    return obj

  if obj instanceof Date
    return new Date(obj.getTime()) 

  if obj instanceof RegExp
    flags = ''
    flags += 'g' if obj.global?
    flags += 'i' if obj.ignoreCase?
    flags += 'm' if obj.multiline?
    flags += 'y' if obj.sticky?
    return new RegExp(obj.source, flags) 

  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

Discussion

The difference between copying an object through assignment and through this clone-function is how they handle references. The assignment 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 assignment copy:

x =
  foo: 'bar'
  bar: 'foo'

y = x

y.foo = 'test'

console.log x.foo isnt y.foo, x.foo, y.foo
# => false, test, test

As you can see, when you change y after the copy, you also change x.