call private function in the constructor of module pattern

Question

This is my first attempt to write a module in java script.

myClass = (function() {
    //private members
    SELF = {};
    //constructor
    (function _init()
    {
        SELF.private_member = 10;
    });

    function _print()
    {
        return SELF.private_member;
    }

    return function() {
       _init();
       this.Print = _print;
    };

})();

var obj = new myClass();

I get a error that function _init() is not defined (chrome). How can this be achieved?

Answer 1

The problem in your code is the way that the init function is declared. The below samples will work.

(A) without the brackets:

myClass = (
function() {
    //private members
    SELF = {};

    //constructor
    //this function is executed every time an object is created
    function _init() {
        SELF.private_member = 10;
    }

    function _print() {
        return SELF.private_member;
    }

    return function() {
       _init();
       this.Print = _print;
    };

})();

(B) The second option is to call the _init() function anonymously like this:

myClass = (function() {
    //private members
    SELF = {};

    //constructor
    (function() {
        SELF.private_member = 10;
    })();

    function _print() {
        return SELF.private_member;
    }

    return function() {
       this.Print = _print;
    };
})();

// Better syntax for example B)
myClass = (function() {
    //private members
    SELF = {};

    function _print() {
        return SELF.private_member;
    }

    //constructor; right before the return statement
    //this code is executed ONCE only
    SELF.private_member = 10;

    return function() {
       //this code is exectuted every time an object is created
       this.Print = _print;
    };
})();

I would use the first example, as sample (B) can be a little tricky: The constructor is called instantly (ie before the function _print() is defined), so you have to take care that any functions/variables that the constructor uses are above the constructor function!

With (A) you don't have this problem.

Functional differences :

(A) The constructor function _init() is called every time you create a new object. Technically this is very close to a real constructor as in other object oriented programming languages.

(B) The constructor code is only called once, when you declare the class. Technically this is not a constructor here but actually some class initialization code.

Answer 2

Couple of questions

1. Why is _init() a constructor? Constructor should be the one which you are returning.
2. Any particular reason you have grouped (named function expression) the _init() function?

Ignoring the answers what you want to get can be achieved in two ways

myClass = (function() {
    //private members
    SELF = {};
    //constructor
    (function _init()
    {
        SELF.private_member = 10;
    })();

    function _print()
    {
        return SELF.private_member;
    }

    return function() {
       this.Print = _print;
    };

})();

var obj = new myClass();

and

myClass = (function() {
    //private members
    SELF = {};
    //constructor
    function _init()
    {
        SELF.private_member = 10;
    }

    function _print()
    {
        return SELF.private_member;
    }

    return function() {
       _init();
       this.Print = _print;
    };

})();

var obj = new myClass();