FAQ

Misc

Q: Can I use negative array indices?

A: No, there is no easy and quick way to allow negative variables b/c array[i] could really be object[property]. Example of the problem:

i: -1
last: array[i]

Read the following issues for more information: #272, #681, #621

Q: Can I use default parameters like func = (param = 1) -> ?

A: No, because they would significantly complicate the grammar and have the potential to look really confusing within function assignments. The following issues highlight the confusion/complications: #30, #16, #92

Q: Why can't I use with?

A: Because Douglas Crockford says so and #344, #620

Classes

Q: Will you support multiple inheritance/mixins/imports/interfaces/traits or any other fancy class extensions?

A: The short answer is no. You can do any of the above using helpers.

Solution (1) courtesy of jashkenas:

extend = (obj, mixin) ->
  for name, method of mixin
    obj[name] = method

include =  (klass, mixin) ->
  extend klass.prototype, mixin

class Button
  onClick: -> # do stuff

include Button, Options
include Button, Events

Solution (2) courtesy of sethaurus:

implementing = (mixins..., classReference) ->
  for mixin in mixins
    for key, value of mixin::
      (classReference::)[key] = value
  classReference

Button = implementing Options, Events, class _Button
  onClick: -> # do stuff

If you want to learn more, please read these issues on the tracker: #218, #327, #344, #452 and #636. They should have all the pros and cons and why mixins are not part of the core language.

Q: Do class methods work with inheritance?

A: No, class methods do not get inherited b/c JS has no prototype chain for class methods, only for object methods. You can add a simple hook to mimic class-method inheritance, however, by defining an "extended" method on your class.

TODO: Add the following

• Executable class bodies
• Private properties

Unsupported features

Q: Will you add feature X where feature X depends on a platform?

A: No, implementation-specific features are not allowed as a policy. Everything that you write in CoffeeScript should be supported and runnable on any current JavaScript implementation (in practice, this means the lowest common denominator is IE6). Thus, features such as the following will not be implemented: getters & setters, yield

Q: What about extending native objects to allow for things such as forEach and Function.bind ?

A: The rule here is to not extend native objects.