Question

34

Implementing private instance variables in Javascript

rated 0 times [ 37] [ 3] / answers: 1 / hits: 17853 / 12 Years ago, mon, march 5, 2012, 12:00:00

I don't know how I've missed this for so long. I've been presuming private instance variables to work like this, but they don't. They're private (as in non-global), certainly, but the variables are shared across instances. This led to some very confusing bugs.

I thought I was following the best practices implemented by some of the best libraries out there, but it seems I missed something.

var Printer = (function(){

    var _word;



    Printer = function(word){

        _word = word;

    }



    _print = function(){

        console.log(_word);

    }



    Printer.prototype = {

        print: _print

    }

    return Printer;

})();



var a = new Printer(Alex);

var b = new Printer(Bob);



a.print(); //Prints Bob (!)

b.print(); //Prints Bob

I have looked at this post, but it doesn't describe a best practice for implementing private instance variables. (is this even the name of what I want?)
Method and variable scoping of private and instance variables in JavaScript

I also looked at this post, but the use of the 'this' keyword is what I used to do. Because it doesn't obfuscate I was trying to avoid it. Is this really the only way?
Implementing instance methods/variables in prototypal inheritance

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campbelljonahb

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4 Years ago

answered 12 Years ago devinjadong · Accepted Answer

You're doing some wonky stuff with that closure. _word needs to be declared in the Printer function, not lost in anonymous-closure land:

function Printer(word) {

    var _word = word;



    this.print = function () {

        console.log(_word);

    }

}



var a = new Printer(Alex);

var b = new Printer(Bob);



a.print(); //Prints Alex

b.print(); //Prints Bob

This keeps _word private, at the expense of creating a new print function on every Printer instance. To cut this cost, you expose _word and use a single print function on the prototype:

function Printer(word) {

    this._word = word;

}



Printer.prototype.print = function () {

    console.log(this._word);

}



var a = new Printer(Alex);

var b = new Printer(Bob);



a.print(); //Prints Alex

b.print(); //Prints Bob

Does it really matter that _word is exposed? Personally, I don't think so, especially given the _ prefix.