Private fields in javascript classes

JavaScript

var Greeter = (function () {
    var name = "jos";
    
    function Greeter(message, nameTest) {
        this.greeting = message;
        
        if (nameTest) {
            alert(nameTest);
            name = nameTest;
        }
    }
    Greeter.prototype.greet = function () {
        return "Hello, " + this.greeting + name;
    };
    return Greeter;
}());
var t = new Greeter("test ", 'jos');
t.greet();

var t2 = new Greeter("test ", 'Dwi');
t2.greet();

alert(t.greet());

alert(t.name);

alert(t2.greet());

alert(t2.name);