app.coffee -------------------------------------------

{VariableTestLayer} = require "VariableTestLayer"
myVarLayer = new VariableTestLayer
myVarLayer.myVar = "external value"

VariableTestLayer.coffee -----------------------------

class exports.VariableTestLayer extends Layer
    constructor: (@options={}) ->
        myVar = if @options.myVar? then @options.myVar else "internal value only"
        print myVar