Assigning class constructor arguments private variables in coffeescript - constructor

Question

I've found that I can use private variables in coffeescript classes like so:
class Book
title = null
numberOfPages = null
constructor: (t, nop) ->
title = t
numberOfPages = nop
ripOutPages: (numPages) -> numberOfPages = numberOfPages - numPages
isEmpty: -> numberOfPages == 0
So my first question is, is this considered a reasonable approach in coffeescript?
And my second question is, is there a neater way to achieve this (i.e. not having to initialise the variables in the class body then assign them in the constructor)?
Thanks,
Tom

Answer 1

If you look at the generated JS code, you will notice that those are more like "private" static class properties, not "private" instance properties:
var Book;
Book = (function() {
var numberOfPages, title;
title = null;
numberOfPages = null;
function Book(t, nop) {
title = t;
numberOfPages = nop;
}
Book.prototype.ripOutPages = function(numPages) {
return numberOfPages = numberOfPages - numPages;
};
Book.prototype.isEmpty = function() {
return numberOfPages === 0;
};
return Book;
})();
Every instance you create shares the same title and numberOfPages variables. So I guess the answer is: No, it's not a suitable approach for what you are trying to do.
JavaScript simply doesn't have "private" properties (yet).

Answer 2

If you really-really want "private" variable, you might want to define your methods as closures:
class Book
constructor: (t, nop) ->
# Those are two local variables:
title = t
numberOfPages = nop
# The following closures will have access to the two variable above:
#title = -> title # <- accessor to title
#numberOfPages = -> numberOfPages # <- accessor to numberoOfPage
#ripOutPages = (numPages) -> numberOfPages = numberOfPages - numPages
isEmpty: -> #numberOfPages() == 0
# ^^^^^^^^^^^^^^^^
# declared "as usual" as it uses accessor to access the "private" variables
# Some tests
lotr = new Book("The Lord of the Rings",1137)
hobbit = new Book("The Hobbit",276)
console.log lotr.title(), lotr.numberOfPages()
console.log hobbit.title(), hobbit.numberOfPages()
lotr.ripOutPages 100
console.log lotr.title(), lotr.numberOfPages()
Producing:
The Lord of the Rings 1137
The Hobbit 276
The Lord of the Rings 1037