CoffeeScript Class Variable

Question

class Example
  constructor: ->
    $.each [1, 2, 3], (key, value) ->
      @test = value
    return @test
  render: ->
    alert @test

example = new Example()
example.render()

I'm using CoffeeScript (+ jQuery) and this is an class example where I would get the value 3 in @test variable. But this does not happen, can you help me?

A constructor can't return anything. An instance of the class is always returned when you instantiate a class. Doesn't have anything to do with your question but I thought I should let you know. — Sandro
– Sandro, Commented Apr 9, 2012 at 13:02
Correction for others reading along … constructors absolutely can return values; they simply must return an object (that is, no primitive values, such as numbers or strings.) (See: es5.github.io/#x13.2.2, steps 9. and 10.) — ELLIOTTCABLE
– ELLIOTTCABLE, Commented Apr 15, 2013 at 23:55

Luan · Accepted Answer · 2012-04-09 12:49:54Z

3

This is a scoping problem: $.each accepts a function, which has it's on scope, thus, your this variable is not the one you expected.

The working code:

class Example
  constructor: ->
    $.each [1, 2, 3], (key, value) =>
      @test = value
    return @test
  render: ->
    alert @test

example = new Example()
example.render()

What changed? Check the arrow on the $.each call, it's now a fat arrow. Fat arrows does the trick of setting a _this variable and using it when you use @... making your scope the one you expected to be.

Check http://coffeescript.org on the section "Function Binding" for more details!

answered Apr 9, 2012 at 12:49

Luan

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