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JavaScript build a constructor of constructors

Here is a simple example of what I want :
var ConstBuilder = function() {
var constructor = function() {} ;
constructor.prototype = {} ;
return constructor ;
} ;
ConstBuilder.prototype = {
add : function(name, value) {
this.prototype[name] = value ;
}
} ;
var A = new ConstBuilder() ;
A.add('test', function() {
console.log('test') ;
}) ;
var a = new A() ;
a.test() ;
This code will fail as A is not an instance of ConstBuilder (because A comes from a returned var constructor = function() {} and won't have the methods defined in its prototype (add).
But this would be useful to modify the super constructor's prototype to have things like :
ConstBuilder.prototype.remove = function(name) {
delete this.prototype[name] ;
} ;
A.remove('test') ;
a.test ; // undefined
Is there a way to have a function as an instance of another ? So this function may implicitely "inherit" all the methods defined in its constructor's prototype.
Or if you have other suggestions, I aim to build modulable constructors - as instances with prototypes are.

Please make sure you have understood the difference between the .prototype property and the internal inheritance-prototype.
The code will fail as A is not an instance of ConstBuilder. Is there a way to have a function as an instance of another?
A is, as every constructor needs to be, a Function. So if you just define your add and remove methods on the Function.prototype, it will work:
Function.prototype.add = function(name, value) {
this.prototype[name] = value;
};
Function.prototype.remove = function(name) {
delete this.prototype[name];
};
function A() {}
A.add('test', function(){console.log('test');});
var a = new A();
a.test(); // test
A.remove('test');
a.test; // undefined
There is no possibility however to let a function inherit from something else than Function.prototype - see Can a JavaScript object have a prototype chain, but also be a function?. If you don't want to modify the native Function.prototype object, you still can use the mixin pattern:
var Constr = (function() {
function add(name, value) {
this.prototype[name] = value;
}
function remove(name) {
delete this.prototype[name];
}
return function mixin(c) {
c.add = add;
c.remove = remove;
return c;
};
})();
var A = Constr(function() {…});
A.add("test", …);
var a = new A();
a.test(); // test
I aim to build modulable constructors
You could use the builder pattern, as you just have seem to tried.
function ConstBuilder() {
this.prototype = {};
};
ConstBuilder.prototype = {
add: function(name, value) {
this.prototype[name] = value;
},
remove: function(name) {
delete this.prototype[name];
},
getConstructor: function() {
var constructor = function() {};
constructor.prototype = this.prototype;
this.prototype.constructor = constructor;
return constructor;
}
};
var A = new ConstBuilder().add('test', function() {
console.log('test');
}).getConstructor();
var a = new A();
a.test(); // test
To remove functions later, you would need to save a reference to the builder.

I think that you are looking for an example of how to do JavaScript's "prototypical inheritance". When JavaScript looks for a property on an object, it first checks the object itself. Next it checks the prototype. However, since everything in JavaScript is an object and the prototype is an object
function Root(){}
Root.prototype.fromRoot = function() { console.log("I'm on Root's prototype."); };
function Child(){}
Child.prototype = new Root();
Child.prototype.fromChild = function() { console.log("I'm on Child's prototype."); };
var r = new Root();
var c = new Child();
r.fromRoot(); // works
c.fromRoot(); // works
c.fromChild(); // works
r.fromChild(); // fails

function a (x,y,construct)
{
if (!construct) return;
this.x=x;
this.y=y;
}
a.prototype.methoda=function ()
{
return x+y;
}
function b (x,y,d,e)
{
a.call (this,x,y,true) //--- this would inherit all own Objects and Properties of a and become own properties of b
this.d=d;
this.e=e;
}
b.prototype=new a (); //--- this would only inherit the prototype, construct becomes false and isnt worked through, which if not would result in adding propertiy x and y to prototype instead of directly to instance of b,
b.prototype.constructor=b;
var test=new b (1,2,3,4);
b.methoda ();
second way
function a (x,y)
{
if (arguments.callee.doNotConstruct) return;
this.x=x;
this.y=y;
}
a.prototype.methoda=function ()
{
return x+y;
}
function b (x,y,d,e)
{
a.call (this,x,y) //--- this would inherit all own Objects and Properties of a and become own properties of b
this.d=d;
this.e=e;
}
a.doNotConstruct=true;
b.prototype=new a (); //--- this would only inherit the prototype, construct becomes false and isnt worked through, which if not would result in adding propertiy x and y to prototype instead of directly to instance of b,
a.doNotConstruct=false;
b.prototype.constructor=b;
var test=new b (1,2,3,4);
b.methoda ();
put this in a function
function prototypeInheritance (inheritor,parent)
{
parent.doNotConstruct=true;
inheritor=new parent ();
inheritor.prototype.constructor=inheritor;
inheritor.parent=parent;
parent.doNotConstruct=false;
}
you can call the parent property with (arguments.callee.parent) in the inheritor constructor and you can check doNotConstruct with arguments.callee.doNotConstruct in the parent constructor

How to properly derive object with private vars using javascript (prototypal) inheritance

I am new to JavaScript's (prototypal) inheritance and I'm trying to learn more about it.
I am using a simple observer pattern as example, in which I want observable objects to be derived from the 'subject' object. This is what I WANT to do:
function subject()
{
var callbacks = {}
this.register = function(name, callback)
{
callbacks[name] = callback;
}
this.unregister = function(name)
{
delete callbacks[name];
}
var trigger = function()
{
var a = arguments;
var t = this;
$.each(callbacks, function(name, callback)
{
callback.apply(t, a);
});
}
}
list.prototype = new subject()
function list()
{
var items = {}
this.add = function(name, item)
{
items[name] = item;
trigger('add', name);
}
this.remove = function(name)
{
delete items[name];
trigger('remove', name);
}
}
Now when using the code above like below, I run into my first problem:
var l = new list()
l.register('observer1', function() { console.log(this, arguments) });
l.add('item1', 'value1'); // <-- ReferenceError: trigger is not defined, trigger('add', name);
To continue testing I made the trigger function 'public' using this.trigger instead. Running my example again I run into the next problem:
var l = new list()
l.register('observer1', function() { console.log(this, arguments) });
l.add('item1', 'value1'); // <-- output: subject, ["add", "item1"]
The this object is subject, I want it to be list. My third problem occurs when creating another list:
var l2 = new list();
//Don;t register any observers
l2.add('item1', 'value1'); // <-- output: subject, ["add", "item1"]
The callbacks list is shared between the 2 lists.
I've tried similar things with Object.create(new subject()) as well and run into similar problems.
My 3 questions in this are:
Can I have private methods that can be used in derived objects (and
should I even care about having them private or public)?
How can I have the this object I want (without needing to use function.call in the derived object, if possible)?
How can I keep the callbacks list in the base object without it being shared?

An interesting question. As for #1 and #2: let's say you have a function foo:
function foo() {
var _private = 'private var!';
this.access = function () {
return _private;
}
}
access is a so-called privileged method, it's a closure that can access the private variable private.
you can inherit the whole thing by making use of call, like so:
function bar() {
foo.call(this);
}
var b = new bar();
console.log(b.output()); // prints 'private var!'
With the methods apply, call and bind you can establish the context of a function, effectively tamper with the this object. (your #2 question, read here )
Naturally you cannot make use of a totally private method in a derived object. You'd need an accessor method which would defeat the purpose of the original method being private. Having said that, that's the way it works in strongly typed languages too (in java if you mark a method as private not even subclases will be able to access it, it would have to be protected).
As for #3, I cannot think of how to keep callbacks shared and private.
But you can make it a static property for all instances of a function (much like a static property in a lanaguage like java) by simply declaring a function like:
function foo() {
}
add your prototypes which will be assigned to each instance
foo.prototype.bar = // ...
and a static property
foo.callbacks = [];
All instances of foo will share the callbacks property.

You can’t have private methods, and that’s that. It will never work both properly and nicely at the same time, so don’t bother trying to emulate them in JavaScript.
Then all you have to do is call the parent’s constructor in the derived constructor.
function subject()
{
var callbacks = {};
this.register = function(name, callback)
{
callbacks[name] = callback;
};
this.unregister = function(name)
{
delete callbacks[name];
};
this.trigger = function()
{
var a = arguments;
var t = this;
$.each(callbacks, function(name, callback)
{
callback.apply(t, a);
});
};
}
list.prototype = Object.create(subject);
list.prototype.constructor = list;
function list()
{
subject.call(this);
var items = {};
this.add = function(name, item)
{
items[name] = item;
this.trigger('add', name);
};
this.remove = function(name)
{
delete items[name];
this.trigger('remove', name);
};
}

Incorporating Joe's suggestion, this is what I eventually ended up with:
function subject()
{
var callbacks = {}
this.register = function(name, callback)
{
callbacks[name] = callback;
}
this.unregister = function(name)
{
delete callbacks[name];
}
trigger = function()
{
var a = arguments;
var t = this;
$.each(callbacks, function(name, callback)
{
callback.apply(t, a);
});
}
}
//without the following line, 'this' in firefox is 'subject' instead of 'list' (in chrome it is)
list.prototype = new subject()
//without these, 'list' is not an instanceof 'subject'
list.constructor = subject;
list.prototype.constructor = list;
function list(n)
{
this.name = n;
subject.call(this); //as suggested by Joe
var items = {}
this.add = function(name, item)
{
items[name] = item;
trigger.call(this, 'add', name); //no way to do this without using call/apply
}
this.remove = function(name)
{
delete items[name];
trigger.call(this, 'remove', name); //no way to do this without using call/apply
}
this.getitems = function() { return items }
}
//without the following line, 'this' in firefox is 'subject' instead of 'queue'
queue.prototype = new subject()
//without these, 'queue' is not an instanceof 'subject'
queue.constructor = subject;
queue.prototype.constructor = queue;
function queue(n)
{
this.name = n;
subject.call(this); //as suggested by Joe
var items = [];
this.enqueue = function(item)
{
items.push(item);
trigger.call(this, 'enqueue', item); //no way to do this without using call/apply
}
this.dequeue = function()
{
var d = items.shift();
trigger.call(this, 'dequeue', d); //no way to do this without using call/apply
return d;
}
this.getitems = function() { return items }
}
var l1 = new list('l1')
l1.register('observer1', function() { console.log('l1', this, arguments) });
l1.add('item1', 'value1');
// ^ 'l1', list { name = 'l1' ... }, ['add', 'item1']
var l2 = new list('l2')
l2.register('observer2', function() { console.log('l2', this, arguments) });
l2.add('item2', 'value2');
// ^ 'l2', list { name = 'l2' ... }, ['add', 'item2']
var q1 = new queue('q1')
q1.register('observer3', function() { console.log('q1', this, arguments) });
q1.enqueue('item3');
// ^ 'q1', queue { name = 'q1' ... }, ['enqueue', 'item3']
console.log(l1 instanceof list, l1 instanceof subject, l1 instanceof queue);
// ^ true, true, false
console.log(q1 instanceof list, q1 instanceof subject, q1 instanceof queue);
// ^ false, true, true
This ticks all of my boxes (except for the use of call, but I can live with that).
Thanks for all the help,
Mattie
EDIT: appearantly this does not work as expected. creating a new object overwrites the other objects callbacks

How do I augment a method from the superclass in javascript

I have a method in a base class that I want to keep in a subclass, but just add to it. I've found lots of stuff on augmenting classes and objects with properties and methods, but I can't find, or don't understand, how to just augment the method. The worst case scenario is that I would have to paste the entire method of the parent class into the subclass, but that seems like duplicate code... please help
function someObject (){
this.someProperty = 1;
this.incrementProperty = function incrementProperty(){
this.propertyOfSomeObject += 1;
}
}
function newObject (){
someObject.call(this);
this.incrementProperty = function incrementProperty(){
//do everything the super class has for this property already
return this.someProperty;
}
}
var incrementer = new newObject;
alert (incrementer.incrementProperty()); //I want output to be 2

// parent object
function someObject () {
this.someProperty = 1;
}
// add incrementProperty to the prototype so you're not creating a new function
// every time you instantiate the object
someObject.prototype.incrementProperty = function() {
this.someProperty += 1;
return this.someProperty;
}
// child object
function newObject () {
// we could do useful work here
}
// setup new object as a child class of someObject
newObject.prototype = new someObject();
// this allows us to use "parent" to call someObject's functions
newObject.prototype.parent = someObject.prototype;
// make sure the constructor points to the right place (not someObject)
newObject.constructor = newObject;
newObject.prototype.incrementProperty = function() {
// do everything the super class has for this property already
this.parent.incrementProperty.call(this);
return this.someProperty;
}
var incrementer = new newObject();
alert (incrementer.incrementProperty()); // I want output to be 2
See: http://jsfiddle.net/J7RhA/

this should do, you have to use prototype to have a real concept of oo with javascript
function someObject (){
this.someProperty = 1;
this.propertyOfSomeObject = 0;
this.incrementProperty = function incrementProperty(){
this.propertyOfSomeObject += 1;
return this.propertyOfSomeObject;
}
}
function newObject (){
someObject.call(this);
this.incrementProperty = function incrementProperty(){
this.__super__.incrementProperty.apply(this);
return this.propertyOfSomeObject + 1;
}
}
newObject.prototype = new someObject()
newObject.prototype.__super__ = newObject.prototype
var incrementer = new newObject();
alert(incrementer.incrementProperty()); //I want output to be 2
experiment removing incrementProperty from newObject and it will return 1

I usually use the augment library to write classes in JavaScript. This is how I would rewrite your code using augment:
var Foo = Object.augment(function () {
this.constructor = function () {
this.someProperty = 1;
};
this.incrementProperty = function () {
this.someProperty++;
};
});
var Bar = Foo.augment(function (base) {
this.constructor = function () {
base.constructor.call(this);
};
this.incrementProperty = function () {
base.incrementProperty.call(this);
return this.someProperty;
};
});
As you can see since Bar extends Foo it gets Foo.prototype as a parameter (which we call base). This allows you to easily call the base class constructor and incrementProperty functions. It also shows that the constructor itself is just another method defined on the prototype.
var bar = new Bar;
alert(bar.incrementProperty());
The output will be 2 as expected. See the demo for yourself: http://jsfiddle.net/47gmQ/

From this answer:
Overriding functions
Sometimes children need to extend parent functions.
You want the 'child' (=RussionMini) to do something extra. When RussionMini can call the Hamster code to do something and then do something extra you don't need to copy and paste Hamster code to RussionMini.
In the following example we assume that a Hamster can run 3km an hour but a Russion mini can only run half as fast. We can hard code 3/2 in RussionMini but if this value were to change we have multiple places in code where it needs changing. Here is how we use Hamster.prototype to get the parent (Hamster) speed.
// from goog.inherits in closure library
var inherits = function(childCtor, parentCtor) {
function tempCtor() {};
tempCtor.prototype = parentCtor.prototype;
childCtor.prototype = new tempCtor();
childCtor.prototype.constructor = childCtor;
};
var Hamster = function(name){
if(name===undefined){
throw new Error("Name cannot be undefined");
}
this.name=name;
}
Hamster.prototype.getSpeed=function(){
return 3;
}
Hamster.prototype.run=function(){
//Russionmini does not need to implement this function as
//it will do exactly the same as it does for Hamster
//But Russionmini does need to implement getSpeed as it
//won't return the same as Hamster (see later in the code)
return "I am running at " +
this.getSpeed() + "km an hour.";
}
var RussionMini=function(name){
Hamster.apply(this,arguments);
}
//call this before setting RussionMini prototypes
inherits(RussionMini,Hamster);
RussionMini.prototype.getSpeed=function(){
return Hamster.prototype
.getSpeed.call(this)/2;
}
var betty=new RussionMini("Betty");
console.log(betty.run());//=I am running at 1.5km an hour.

Pattern for enforcing new in javascript

I have been reading JavaScript Patterns book by Stoyan Stefanov and one of the patterns to enforcing the new operator for constructor functions goes like this
function Waffle() {
if (!(this instanceof Waffle)) {
return new Waffle();
}
this.tastes = "yummy";
}
Waffle.prototype.wantAnother = true;
when writing this way you can invoke Waffle either one of these ways
var first = new Waffle(),
second = Waffle();
I think this is a helpful feature not sure if it's implemented in future versions of ecma/javascript
I came up with something on my own that I thought could just copy and paste each time when creating a constructor function
something like this
function checkInstance (name) {
if (name.constructor.name === undefined) {
return "construct it"
} else {
return false;
}
}
function Waffle() {
var _self = checkInstance.call(this, this);
if (_self === "construct it") {
return new Waffle()
}
this.tastes = "yummy"
}
var waffle = Waffle()
waffle
Therefore I can invoke Waffle either way new Waffle or Waffle() and still have it return an object
My problem that I'm having is here
if (_self === "construct it") {
return new Waffle()
}
Is there anyway I can refer to new Waffle() without referring to the actual name of the constructor function meaning so I could copy and paste this each time and not have to change anything. Meaning I could I save Waffle() as a variable and do something like
return new var
I wish I could use this.name but that doesn't work either until it is invoked.
I have a feeling I can't but wanted to at least ask some of the people here on stack overflow if it was a possibility
Again your comments and feedback is appreciated

I have a better solution. This is what you're currently doing:
function Waffle() {
if (!(this instanceof Waffle))
return new Waffle;
this.tastes = "yummy";
}
Waffle.prototype.wantAnother = true;
This pattern isn't really nice because you're mixing the code to construct a new object with the code to check if the new keyword is being used.
I've mentioned before that you shouldn't use the new keyword in JavaScript as it breaks functional features. Instead let's create another function which does the same thing:
Function.prototype.new = (function () {
return function () {
functor.prototype = this.prototype;
return new functor(this, arguments);
};
function functor(constructor, args) {
return constructor.apply(this, args);
}
}());
This function allows you to create an instance of a function as follows:
var waffle = Waffle.new();
However we don't want to use new at all. So to do away with it we'll create a function which wraps a constructor as follows:
function constructible(constructor) {
function functor() { return Function.new.apply(constructor, arguments); }
functor.prototype = constructor.prototype;
return functor;
}
Now we can define the Waffle function as follows:
var Waffle = constructible(function () {
this.tastes = "yummy";
});
Waffle.prototype.wantAnother = true;
Now you can create objects with or without using new:
var first = new Waffle;
var second = Waffle();
Note: The constructible function is pretty slow. Use the following version of constructible instead - it's a little faster:
function constructible(constructor) {
constructor = Function.bind.bind(constructor, null);
function functor() { return new (constructor.apply(null, arguments)); }
functor.prototype = constructor.prototype;
return functor;
}
Personally I wouldn't use either of these two methods. I would just remember to write new, or (more likely) I would restructure my code as follows:
var waffle = {
create: function () {
var waffle = Object.create(this);
waffle.tastes = "yummy";
return waffle;
},
wantAnother: true
};
var first = waffle.create();
var second = waffle.create();
If you want to know more about this pattern then read the following answer: https://stackoverflow.com/a/17008403/783743

You could use something like this:
var Waffle = (function() {
function Waffle() {
this.tastes = "yummy"
}
return exportCtor( Waffle );
})();
var waffle = Waffle();
alert(waffle.tastes);
console.log(Waffle);
/*
function ConstructorProxy() {
"use strict";
return new Constructor();
}
*/
http://jsfiddle.net/ywQJF/
It handles variable arguments too

arguments.callee, which refers to the current function, is the most simple solution. It's is deprecated, though, so use it at your own risk.
function Waffle() {
if (!(this instanceof arguments.callee))
return new arguments.callee();
this.tastes = 'yummy';
}
It's a hard problem also because you probably want to preserve the arguments you're passing, as Vinothbabu mentioned. But if you real intention is enforcing new, you could simply throw an error, which is a simple two lines of code:
if (!(this instanceof Waffle))
throw new Error('Constructor called without new');
You could even wrap it in a function:
function cons(C) {
var c = function () {
if (!(this instanceof c))
throw new Error('Constructor called without new');
C.apply(this, arguments);
};
c.prototype = C.prototype;
return c;
}
var Waffle = cons(function () {
this.tastes = 'yummy';
});
Waffle.prototype.wantAnother = function () {
return true;
};
new Waffle(); // { tastes: 'yummy', 'wantAnother': true }
Waffle(); // throws error
Now Waffle must be called with new -- otherwise, it throws an error.

The best approach, in my opinion, is not to enable yourself to invoke things incorrectly:
function Waffle() {
if (!(this instanceof Waffle)) {
throw "Waffles need to be fresh or they're gross. Use 'new'.";
}
}
But, if you simply must enable yourself to write inconsistent code, make initialization a separate step.
function Waffle(options) {
var o = options || {};
if (this instanceof Waffle) {
this.init = function() {
/* this is really your constructor */
console.log("initializing ... ");
}
if (!o.__do_not_initialize) {
this.init(arguments);
}
} else {
var rv = new Waffle( { __do_not_initialize: true } );
rv.init(arguments);
return rv;
}
}
If you want to force consistency the other way -- never using the new keyword, create a builder function:
function BuildWaffle(options) {
var o = options || {};
if (this instanceof WaffleBuilder) {
throw "BuildWaffle cannot be instantiated.";
}
var Waffle = function Waffle() { /* whatever */ }
Waffle.prototype.doStuff = function() { /* whatever else */ }
var rv = new Waffle(options);
return rv;
}

There is simpler way how to enforce creation of new object even without new:
function Waffle() {
return {tastes:"yummy"};
}
var a = Waffle();
var b = new Waffle();
alert(a.tastes); // yummy
alert(b.tastes); // yummy
Explanation
Using new with function, there are two possibilities:
the function returns object: the object is the result of the new function() expression
the function doesn't return object: the function itself with new context is returned
See the ECMA script documentation
Workaround: prototype and arguments
function Waffle(taste,how) {
return {
tastes: taste+" "+how,
__proto__: Waffle.prototype
}
}
Waffle.prototype.wantmore = "yes";
var a = Waffle("yummy","much");
var b = new Waffle("gummy","little");
console.log(a.tastes,b.tastes); // yummy much, gummy little
console.log(a.wantmore,b.wantmore); // yes, yes
This deserves a fiddle.
Note: constructor.name (which you used in your pattern) is not standard
Note 2: __proto__ is also not standard, but is supported by modern browsers and will be standardized in ES6.

if (!(this instanceof Waffle)) {
return new Waffle();
}
This has two problems...
one that it won 't work in an anonymous function which has no name
it loses all arguments sent to the constructor.
Using a more generic approach might look something more like this:
if (!instanceExists(this, arguments)) {
return requireInstance(this, arguments);
}
This approach ensures that the constructor is called with new, without having to state the function'
s name, andadds all arguments sent to the constuctor so they aren 't lost during the process.
Here 's the full code for the above:
Function.prototype.callNew = function (args) {
var a = [];
for (var i = 0; i < args.length; i++) a.push("a[" + i + "]");
var fn = new Function("var a=arguments;return new this(" + a.join(",") + ");");
return fn.apply(this, args);
}
function instanceExists(t, args) {
if (t instanceof args.callee) {
return true;
} else {
return false;
}
}
function requireInstance(t, args) {
var fn = args.callee;
if (!instanceExists(t, args)) {
return fn.callNew(args);
}
}
function Waffle(one, two, three) {
if (!instanceExists(this, arguments)) {
return requireInstance(this, arguments);
}
this.one = one;
this.two = two;
this.three = three;
}
Waffle.prototype.serve = function () {
var out = [];
for (var j in this) {
if (!this.hasOwnProperty(j)) continue;
out.push(j + ': ' + this[j]);
}
return ' {
' + out.join(",\n") + '
}
';
}
A fiddle for you to play with.
http://jsfiddle.net/RkPpH/
var waffle = Waffle(1, 2, 3);
alert(waffle.serve());

I didn't get a sense of whether this was client or server-side, but a pattern I use sometimes goes as follows. I use this in Node but have attempted to make it a possible client-side solution as well - the Node-specific stuff is commented out but there for reference depending on your environment.
First, I create something to be used along the lines of a traditional OO base or super class like so:
//// Node:
//module.exports.Base = Base;
function Base(opts) {
var self = this;
if (!(self instanceof Base)) return new Base(opts);
self.opts = opts || {};
}
Upon which you can define your methods, in usual the fashion. You can even manually throw if the method should be provided by subclasses implementing something like abstract:
// commonMethod is available to subclasses:
Base.prototype.commonMethod = function () {
var self = this;
//access self.opts to get the constructor arguments.
//makes self always point to the right object.
}
// Provide abstractMethod, but subclass is responsible for implementation:
Base.prototype.abstractMethod = function () {
//or throw an error if this should be implemented by subclasses:
throw new Error('implement me');
}
Now you can do this:
//// If using Node:
//var inherits = require('util').inherits;
//var Parent = require('./Base').Base;
function Sub (opts) {
var self = this;
//// If using node and you want super_ to be called prior to creating a new Sub:
//if(Sub.super_) Sub.super_.call(this, opts);
// Always do this:
if (!(self instanceof Sub)) return new Sub(opts);
//// If using node and you are ok with super_ called after creating new Sub:
//if(Sub.super_) Sub.super_.call(this, opts);
//// otherwise:
parent(opts);
}
//// If using Node:
//inherits(Sub, Base);
//// Otherwise:
Sub.prototype.constructor = Base;
Sub.prototype.parent = Base.prototype;
//and provide the implementation of abstractMethod:
Sub.prototype.abstractMethod() {
//...
}
And to formally answer the specific question, all of the
if (!(self instanceof Sub)) return new Sub(opts);
is where you get the guaranteed new situation.

How to make a JavaScript singleton with a constructor without using return?

I currently know two ways to construct singletons in JavaScript. First:
var singleton = {
publicVariable: "I'm public",
publicMethod: function() {}
};
It is perfect except that it does not have a constructor where I could run initialization code.
Second:
(function() {
var privateVariable = "I'm private";
var privateFunction = function() {}
return {
publicVariable: "I'm public",
publicMethod: function () {}
}
})();
The first version does not have private properties nor does it have a constructor, but it is faster and simpler. The second version is more complex, ugly, but has a constructor and private properties.
I'm not in a need for private properties, I just want to have a constructor. Is there something I am missing or are the two approaches above the only ones I've got?

function Singleton() {
if ( Singleton.instance )
return Singleton.instance;
Singleton.instance = this;
this.prop1 = 5;
this.method = function() {};
}​

Here is my solution with closures:
function Singleton() {
Singleton.getInstance = (function(_this) {
return function() { return _this; };
})(this);
}
Test:
var foo = new Singleton();
var bar = Singleton.getInstance();
foo === bar; // true

If you are just looking for a place to initialise your singleton, how about this?
var singleton = {
'pubvar': null,
'init': function() {
this.pubvar = 'I am public!';
return this;
}
}.init();
console.assert(singleton.pubvar === 'I am public!');
Simple and elegant.

var singleton = new function() { // <<----Notice the new here
//constructorcode....
this.publicproperty ="blabla";
}
This is basically the same as creating a function, then instantly assiging a new instace of it to the variable singleton. Like var singleton = new SingletonObject();
I highly advice against using singletons this way in javscript though because of the execution order is based on where in the file you place the object and not on your own logic.

What about this?
var Singleton = (function() {
var instance;
// this is actual constructor with params
return function(cfg) {
if (typeof instance == 'undefined') {
instance = this;
this.cfg = cfg;
}
return instance;
};
})();
var a = new Singleton('a');
var b = new Singleton('b');
//a === b; <-- true
//a.cfg <-- 'a'
//b.cfg <-- 'a'

I make it an actual Singleton with static functions and no this like so:
class S {
//"constructor"
static init() {
//Note: Since it's a singleton, there's no "this" instance.
//Instead, you store variables directly on the class.
S.myVar = 7;
}
static myOtherFunc() {
alert(S.myVar);
}
}
//Immediately call init() to make it the "constructor".
//Alternatively, you can call init() elsewhere if you'd
//like to initialize it at a particular time.
S.init();
//Later:
S.myOtherFunc();
S.myVar = 10;