Implementing private instance variables in Javascript

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