Constructors?

example1.js

// method as a constructor.

function MyClass () {
	this.isInitialized = false;
}

MyClass.prototype.initialize = function () {
	this.initialized = true;
};

var myInstance = new MyClass();
var otherInstance = new myInstance.initialize();

console.log(otherInstance.initialized); // true
console.log(otherInstance instanceof myInstance.initialize); // true

example2.js

// a function that needs to be called with `new`, but isn't a constructor.

function MyClass () {
    if ( !( this instanceof MyClass ) ) {
        throw new Error("Must be called with new.");
    }
    
    return Object.create(null);
}

var myInstance = new MyClass();
console.log(myInstance instanceof MyClass); // false

example3.js

// a constructor that can't be called with new.

function MyClass () {
    if ( this instanceof MyClass ) {
        throw new Error("cannot be called with new");
    }
    
    return Object.create(MyClass.prototype);
}

var myInstance = MyClass();
console.log(myInstance instanceof MyClass); // true

example4.js

// a function that returns the constructed object, which however isn't an instance of MyClass.

function MyClass () {
    this.__proto__ = {};
}

var myInstance = new MyClass();
console.log(myInstance instanceof MyClass); // false