Implementing private instance variables in Javascript

Question

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), cer

Answer 1

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.