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 due to platform specificity

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). The following represents a non-inclusive list of features that will not be implemented:

• forEach
• getters & setters
• Function#bind
• yield