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), 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

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.