coffeeescript: how to declare private function that can be used from within Array.reduce?

Question

The main objective is to filter duplicate items from an array by any given property. The solution I'm trying to use is in js @ https://stackoverflow.com/a/31194441/618220

I tried to implement it in coffeescript. It is all good, except the scoping of the functions. I don't want _indexOfProperty function to be called from outside - since it is useless in all other contexts. But if I make it private (by removing @ in declaration), I cannot call it from within inputArray.reduce

My coffee code looks like this:

Utils = ->
    @filterItemsByProperty= (inputArray,property)=>
        if not _.isArray inputArray
            return inputArray
        r = inputArray.reduce(((a,b,c,d,e)=>
                if @._indexOfProperty(a,b,property) < 0
                    a.push(b)
                a
            ),[])
        r

    @_indexOfProperty= (a,b,prop) ->
        i = 0
        while i< a.length
            if a[i][prop] == b[prop]
                return i
            i++
        -1

    return

window.utils = Utils

here's how I invoke it from other places:

App.utils.filterItemsByProperty(personArray,"name")

And right now, anyone can do this as well:

App.utils._indexOfProperty(1,2,3)

How to modify the coffee to stop this ?

score 2 · Answer 1 · answered Sep 14 '15 at 12:44

2

Just don't put _indexOfProperty on this / @ and it won't be visible:

Utils = ->
    _indexOfProperty = (a,b,prop) ->
        i = 0
        while i< a.length
            if a[i][prop] == b[prop]
                return i
            i++
        -1

    @filterItemsByProperty= (inputArray,property)=>
        if not _.isArray inputArray
            return inputArray
        r = inputArray.reduce(((a,b,c,d,e)=>
                if _indexOfProperty(a,b,property) < 0
                    a.push(b)
                a
            ),[])
        r

    return

window.utils = Utils

answered Sep 14 '15 at 12:44

ccg

313
2
8

Thanks, it worked, and I'm surprised. As already mentioned in the question, I know that removing @ will make the function private. But I'm unable to figure out why I could not access it from within .reduce earlier today? Will have to check tomorrow. – MrClan Sep 14 '15 at 14:37
@MrClan sorry, I must have read over it. Probably you still had the `@` symbol in your reducer function or you might have had a typo :) – ccg Sep 14 '15 at 14:46

natdico · Accepted Answer · 2015-09-14T12:49:52.927

Can you please remove the '@', and this time define a local variable "indexOfProperty" within filterItemsByProperty scope and assign to it "_indexOfProperty" (that way you can use "indexOfProperty" within reduce())?

@filterItemsByProperty = (inputArray, property) ->
    indexOfProperty = _indexOfProperty
    if !_.isArray(inputArray)
      return inputArray
    r = inputArray.reduce(((a, b, c, d, e) ->
      if indexOfProperty(a, b, property) < 0
        a.push b
        a
      ), [])
    r

coffeeescript: how to declare private function that can be used from within Array.reduce?

2 Answers2