CoffeeScript Private Class Instance Variables

Question

I have the following CoffeeScript code:

class Person
  secret = 0
  constructor: (@name, @age, @alive) ->
  inc: -> secret++

Which compiles to the following JavaScript code:

var Person;
Person = (function() {   
    var secret;
    secret = 0;

    function Person(name, age, alive) {
        this.name = name;
        this.age = age;
        this.alive = alive;
    }
    Person.prototype.inc = function() {
        return secret++;
    };
    return Person;
})();

Currently secret is shared between all instances of Person. Is there a way to make secret a private instance variable in CoffeeScript?

Answer 1

I figured out a solution. I am not sure if this is the best solution, so I am still open to others.

CoffeeScript:

class Person
  constructor: (@name, @age, @alive) ->
    secret = 0
    @inc = -> secret++;

JavaScript:

var Person;
Person = (function() {
    function Person(name, age, alive) {
        var secret;
        this.name = name;
        this.age = age;
        this.alive = alive;
        secret = 0;
        this.inc = function() {
            return secret++;
        };
    }
    return Person;
})();

Answer 2

There is no concept of private members in CoffeeScript as there is none in JavaScript. There are local variables, which you do utilize well in your own solution, but while your solution does hide the secret variable from anything outside of a constructor function, it also introduces an overhead of redeclaring the inc method on every instantiation of a class Person.

A mistake very common in JavaScript community is trying to project inexistent features of other languages on it, which an attempt of mimicing private members is obviously a case of. There's no such concept in it and thinking deeper you'll conclude that it would be just unnatural to an extremely loose dynamic environment which JavaScript is.

So don't waste your time and performance of your application on implementing inexistent constructs. Just concentrate on solving your problem - not the problems of lacking language features.

Now ask yourself: what is so hurtful in having all members public?

Taking everything said into account the ultimate solution would be:

class Person
  constructor: (@name, @age, @alive) ->
    @secret = 0
  inc: -> @secret++

Answer 3

While it won't truly hide them, the convention is to prefix "private" members with an underscore. The idea here is that folks consuming this should assume that such members are implementation details and are advised not to use them.