Two nice helper functions for creating constructors in JavaScript.

getinstance.js

// Polyfill for Object.create
function create(proto) {
    function Ctor() {}
    Ctor.prototype = proto;
    return new Ctor();
}

// Always returns an instance of constructor
function getInstance(self, constructor) {
    return self instanceof constructor ? self : create(constructor.prototype);
}

// Our custom constructor
function Container(value) {
    var self = getInstance(this, Container);
    self.value = value;
    return self;
}

var a = new Container(100);
var b = Container(100);

console.log(a.value); // 100
console.log(b.value); // 100