Extend class and pass all constructor arguments to super

Question

How would you extend a class using CoffeeScript, but have the construction arguments passed to super?

Eg:

class List extends Array
    # Some other stuff to make it work...

list = new List(1,2,3)

console.log list

[1, 2, 3]

Answer 1

In general, this would work without additional code; the parent constructor is used unless expressly overridden:

class A
  constructor: ->
    console.log arg for arg in arguments

class B extends A

new B('foo') # output: 'foo'

And the problem isn't that Array doesn't have a constructor method:

coffee> Array.constructor
[Function: Function]

The problem is just that Array is just plain weird. While arrays are "just objects" in principle, in practice they're stored differently. So when you try to apply that constructor to an object that isn't an array (even if it passes the instanceof Array test), it doesn't work.

So, you can use Acorn's solution, but then you may run into other problems down the road (especially if you pass a List to something that expects a true array). For that reason, I'd recommend implementing List as a wrapper around an array instance, rather than trying to use inheritance from a native object type.

While we're on the subject, one very important clarification: When you use super by itself, that does pass all arguments! This behavior is borrowed from Ruby. So

class B extends A
  constructor: ->
    super

will pass along all arguments to A's constructor, while

class B extends A
  constructor: ->
    super()

will invoke A's constructor with no arguments.