The main difference between fat and thin arrows for instance methods in CoffeeScript

fat-vs-thin-arrows.coffee

# Try running this in http://coffeescript.org 's "Try CoffeeScript" tab while having the DevTools console open.
class Foo
  thin: (arg) ->
    console.log 'thin', this, @value?(), arg

  fat: (arg) =>
    console.log 'fat', this, @value?(), arg

  value: ->
    'foobar'

foo = new Foo

# The behaviour is exactly the same in this case:
foo.thin('plain')
foo.fat('plain')

# The behaviour is different when the context of `this` is changed.
# This commonly happens when the functions are used as callbacks.
# Sometimes we use `_.bind`, like below, to achieve that with
# thin-arrow instance methods:
#
#   $('#thin').on 'click', _.bind(foo.thin, foo)
#
# When you run the following, notice that in thin's console log,
# the value of `this` is `window` (hence `@value` is `undefined`),
# and that in fat's console log, the value of `this` is `foo`.
foo.thin.apply(window, ['applied'])
foo.fat.apply(window, ['applied'])