Question

I’m trying to create methods dynamically in coffee script, but as my code shows, the iterator I use to create the methods doesn’t reset its variables between iterations and so I en up with shared variables that conflicts:

class MyClass
  constructor: (@name) ->

  for k, v of ['get', 'set']
    console.log('creating method: ' + v)
    MyClass::[v] = (args...) ->
      method = v
      console.log('executing method: ' + method)

o = new MyClass('dummy')
o.get()
o.set()

Outputs:

> creating method: get
> creating method: set
> executing method: set
> executing method: set

one knows what I’m doing wrong?

Answer 1

Your inner function:

(args...) ->
  method = v
  console.log('executing method: ' + method)

is actually a closure on v so when it is executed, v will evaluate to its last value in the loop (i.e. set). Looks like you just need a bit of closure busting:

build_method = (v) ->
  (args...) ->
    method = v
    console.log('executing method: ' + method)

And then:

for k, v of ['get', 'set']
  console.log('creating method: ' + v)
  MyClass::[v] = build_method(v)

Keep in mind that CoffeeScript is just JavaScript with different makeup so it suffers from a lot of the same problems (as does any language with closures) and those problems have the same solutions.