static property inheritance CoffeeScript

Question

Let's define this simple code :

class Foo
  @foo = 'blah'
  console.log(@foo)

class Bar extends Foo
  constructor: () ->
    console.log(@foo)

  bar: () ->
    console.log(@foo)

b = new Bar
b.bar()

And result is :

blah
undefined
undefined

How can I access @foo in inherited class ?

Answer 1

You actually want to write

console.log(@constructor.foo)

in Bar's constructor. (Working example here.) @constructor points to the class (Bar), which inherits the static properties of Foo. Those properties aren't on the instance, which is what @ points to from the constructor.

(Yes, it's weird that it's @constructor rather than @class, but that's because obj.constructor is a JavaScript-ism, not a special CoffeeScript syntax.)

To clarify further: In the class body, @ points to the class. In the constructor, @ points to the instance. Hence the apparent inconsistency. I devote a lot of time to this in the chapter on classes in my book, CoffeeScript: Accelerated JavaScript Development.

Answer 2

foo is a property of the Foo constructor, not its prototype:

class Bar extends Foo
  constructor: () ->
    console.log(Foo.foo)
  bar: () ->
    console.log(Foo.foo)