creating objects from JS closure: should i use the "new" keyword?

Question

i answered one question about closures here in SO with this sample:

function Constructor() {
    var privateProperty = 'private';
    var privateMethod = function(){
        alert('called from public method');
    };
    return {
        publicProperty: 'im public',
        publicMethod: function(){
            alert('called from public method');
        },
        getter: privateMethod
    }
}

var myObj = new Constructor();

//public
var pubProp = myObj.publicProperty;
myObj.publicMethod();
myObj.getter();

//private - will cause errors
myObj.privateProperty
myObj.privateMethod

a user commented on my answer saying:

Also, if your function explicitly returns an object it is not a good practice to call it with new because that is misleading - if using new you'd expect the result to be an instance of Constructor

i usually create objects using new. but why is it not a good practice? it seems like using new and not using new returns the same thing. what is the proper way of creating objects from closures?

Answer 1

No, it's not the same thing. Consider when using instanceof:

function C1() {
    return {};
}

function C2() {
}

var c1 = new C1();
var c2 = new C2();
alert(c1 instanceof C1); // false; wha...?
alert(c2 instanceof C2); // true, as you'd expect.

Here's a demo.

So instead, create them by assigning to this, possibly with a safeguard to prevent forgotten news.

function Constructor() {
    var privateProperty = 'private';
    var privateMethod = function() {
        alert('Called from private method');
    };

    this.publicProperty = "I'm public!";
    this.publicMethod = function() {
        alert('Called from public method');
    };
    this.getter = privateMethod;
}

Even better, use the prototype when possible:

function Constructor() {
    var privateProperty = 'private';
    var privateMethod = function() {
        alert('Called from private method');
    };

    this.getter = privateMethod;
}

Constructor.prototype.publicProperty = "I'm public!";
Constructor.prototype.publicMethod = function() {
    alert('Called from public method');
};