Develop Reference

access singleton and inner classes each other in javascript

I'm kind of new to javascript. I'm so confused that javascript Objects!!
My code skeleton is bellow...
var jCanvas = new function(){
this.init = function(canvasID){
...
};
var DrawingManager = new function(){
drawInfos = []; // DrawInfo objects will be pushed into this
this.mouseState = MouseState.released;
...
};
function DrawInfo(bm, cl, id, x, y){
...
}
function Point(x, y){
...
}
var MouseState = new function(){
...
};
var Color = new function(){
...
};
var BrushMode = new function(){
...
};
};
I want jCanvas to be singleton class Object.
in jCanvas object, there are many singleton classes such as DrawingManager, MouseState, Color, BrushMode. And 2 more classes which are not singleton classes(Point, DrawInfo)
What I want is that, in DrawingManager, I want to access other classes and singleton class objects.
Problem is browser gives error that "MouseState is undefined".
I think I'm too familiar with Java, C# etc... I want my program to have good structure but this javascript make me so confused and don't know how to make good design pattern..
Please help me out..

To declare functions, don't use the new keyword. Only use it when creating instances of objects.
In JavaScript, you can declare a "class" like this (the body of the function is the constructor):
function MyClass (arg1) {
this.myClassProperty = arg1;
}
And then instantiate it:
var myObj = new MyClass();
If you want to create a singleton, the best method is to use an immediately invoked function:
var MySingleton = (function() {
var myPrivateFunction = function() { ... };
var myExportedFunction = function() { ... };
var myPrivateVar = 5;
return {
myExportedFunction: myExportedFunction
};
})();
Here, you create an anonymous function and immediately run it. It is kind of a more advanced concept though.
Or you can simply create an object:
var MySingleton = {
myProperty: 1,
myFunction: function() { ... },
...
};

Singleton classes in JavaScript make no sense. Either make a constructor ("class" for Java people) to instantiate multiple objects, or make an object. There is no point in making a constructor that you will only ever use once, then have the code to sanity-check whether or not you actually do use it only once. Just make an object.
The reason for the error is probably (but I might be wrong, I'm guessing about the rest of your code) the misunderstanding between var x = function ... and function name() ... forms. To whit:
var a = function() { console.log("a"); }
function b() { console.log("b"); }
a(); // a
b(); // b
c(); // c
d(); // TypeError: d is not a function
function c() { console.log("c"); }
var d = function() { console.log("d"); }
They are identical in effect, but they differ in whether they are hoisted to the top of the scope or not. var d is hoisted, just like function c() { ... } - so the variable d will exist, but will be undefined, since the assignment is not hoisted. Having both styles of function declarations is inconsistent unless you have a good reason for it; pick one of them and stick to it, is what I'd recommend.

JavaScript extend objects and prototypes

I am creating a framework to simplify the oriented object coding with prototypes. But I am pondering with inheritance in JavaScript.
By default, to extend an object, we write :
var B = function() { /*...*/ } ;
B.prototype = new A() ;
But what about A constructor function requires a parameter ?
var A = function(args) {
if (!args) throw "Arguments required." ;
} ;
Or maybe A constructor function could also perform unwanted things before B was instancied.
What would you suggest to replace the default inheritance behaviour ?
(I thought about storing all the members of all "classes" to copy while inheriting or mixins.)

If you want to inherit from a prototype without calling the constructor, you can use Object.create() to do something like this:
var B = function() { /*...*/ };
B.prototype = Object.create(A.prototype);
B.prototype.constructor = B;
In the above, Object.create(A.prototype) will return a new object whose prototype is given by A.prototype, and it does this without calling A(). The second line is there so you can look up the constructor property on any instances of B, and it'll point back to B().
One thing to note is that Object.create() is relatively new, so you might need a polyfill for older browsers. You can find one here, along with more info:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/create

I usually use a defclass utility function to define "classes" in JavaScript:
function defclass(base, body) {
var uber = base.prototype;
var prototype = Object.create(uber);
var constructor = (body.call(prototype, uber), prototype.constructor);
constructor.prototype = prototype;
return constructor;
}
Then I use it as follows:
var A = defclass(Object, function () {
this.constructor: function (arg1, arg2) {
this.arg1 = arg1;
this.arg2 = arg2;
}
this.log = function (which) {
console.log(which ? this.arg1 : this.arg2);
};
});
Inheritance is dead simple:
var B = defclass(A, function (uber) {
this.constructor = function (arg1, arg2, arg3) {
uber.constructor.call(this, arg1, arg2);
this.arg3 = arg3;
};
this.log = function (which) {
uber.log.call(this, which);
console.log(this.arg3);
};
});
As you can see when we are extending a "class" we use Object.create. This is the new way of inheritance. Using new is antiquated. In the constructor of B we pass arguments to the constructor of A using uber.constructor.call.
If you like this pattern then you should take a look at the augment library.

Why won't this object's prototype's init function run

I use the following function for creating new objects.
function newObj(o) {
var params = Array.prototype.slice.call(arguments,1);
function F() {}
F.prototype = o;
var obj = new F();
if(params.length) {
obj.init.apply(obj,params);
}
return obj;
}
And it works well most of the time. However one of my base "classes" is now defined as inheriting from another base class
SPZ.EditablePuzzle = function () {
// function and variable definitions
return {
///some methods and properties
}
}();
SPZ.EditablePuzzle.prototype = SPZ.Puzzle;
Now when I use newObj() to create a new SPZ.EditablePuzzle the init function is not defined even though it is defined in SPZ.Puzzle and I make sure EditablePuzzle runs after Puzzle
Why won't my newObj function find the init function? Shouldn't it automatically look in the prototype as soon as it fails to find it in the object itself?

I suspect the inheritance is not well set. try doing
SPZ.EditablePuzzle.prototype = new SPZ.Puzzle;
Might solve this problem, though I am not sure.

Concerning the problem:
function Foo() {
}
Foo.prototype.init = function() {
console.log('bla');
};
function FooBar() {
}
FooBar.prototype = Foo; // looks fishy...
var kitten = new FooBar();
console.log(kitten.init); // yields undefined, uh what?
The problem is, that in this case Foo itself gets assigned to the prototype property, when in fact you wanted to do:
FooBar.prototype = Foo.prototype
See the difference? Foo has no init property as it is defined on the prototype object.

Is there a way to change the prototype from within a function in Javascript?

I have this type of code for prototypal inheritance in Javascript.
function A() {
this.a = 1;
}
function B() {
someEl.innerHTML(this.a);
}
B.prototype = new A();
However, I am wondering if there is a better way to do it, preferably one that brings the prototype declaration into the declaration of B.
I tried setting this.prototype in B, but it just showed this.a as undefined.
So, is there a better way to do this?

The prototype property should be used only in constructor functions, this.prototype doesn't makes sense because this just the new object instance, not the constructor.
Assign the prototype inside the constructor itself isn't a good idea, since that assignment will be executed each time you create a new object (by new B();).
Your code seems perfectly valid to me, the only thing that you should be aware of, is that if you replace the prototype property of a constructor, like you do it with B.prototype = new A(); the constructor property of the object instances of B, will point to A.
Usually is recommended to restore that property, e.g.:
function A() {
this.a = 1;
}
function B() {
someEl.innerHTML(this.a);
}
B.prototype = new A();
B.prototype.constructor = B; // restore the `constructor` property
var foo = new B;
foo.constructor === B; // true
Give a look to:
Constructors considered mildly confusing (article)
Setting javascript prototype function within object class declaration (SO question)

Try:
function A() {
this.a = 1;
}
function B() {
this.prototype = new A();
someEl.innerHTML(this.A.a); //notice A.a
}

function MyClass() {
this.initialize();
}
MyClass.prototype = {
initialize: function() { // adding a function name
this.some_var = "Blue"; // makes debugging easier
},
tell_color: function() {
alert(this.color);
},
color: "red"
}
var instance = new MyClass();
instance.tell_color();
This wasn't my idea, but I don't know anymore where I've found it.

Inheritance in JS is a little weird, but everyone else is correct that you should never put a prototype within a constructor. The point of prototyped functions/vars is so you only have a singular instance of that function for ALL instances of said class to use. Likewise it is good for class vars, a singular value which every instance will reference.
Really, you should make class B inherit from class A:
function B(){
this.object = new A();
}
B.prototype.a = function(){
return object.a;
}
which can be a little annoying since you must then create new getters/setters, buuuuuuuttttt, if you are feeling a little adventurous, you can build something like this......
function breakApart(what){
var functions = [];
var properties = [];
for(property in what){
if(typeof what[property] == 'function'){
functions.push({'name':property,'value':what[property]});
}else{
properties.push({'name':property,'value':what[property]});
}
}
var data = [
{'title':'functions','data':functions},
{'title':'properties', 'data':properties}
];
return data;
}
Add a few eval() calls and some logic and you can make this function build you new getters/setters/function references.

javascript singleton question

I just read a few threads on the discussion of singleton design in javascript. I'm 100% new to the Design Pattern stuff but as I see since a Singleton by definition won't have the need to be instantiated, conceptually if it's not to be instantiated, in my opinion it doesn't have to be treated like conventional objects which are created from a blueprint(classes). So my wonder is why not just think of a singleton just as something statically available that is wrapped in some sort of scope and that should be all.
From the threads I saw, most of them make a singleton though traditional javascript
new function(){}
followed by making a pseudo constructor.
Well I just think an object literal is enough enough:
var singleton = {
dothis: function(){},
dothat: function(){}
}
right? Or anybody got better insights?
[update] : Again my point is why don't people just use a simpler way to make singletons in javascript as I showed in the second snippet, if there's an absolute reason please tell me. I'm usually afraid of this kind of situation that I simplify things to much :D

I agree with you, the simplest way is to use a object literal, but if you want private members, you could implement taking advantage of closures:
var myInstance = (function() {
var privateVar;
function privateMethod () {
// ...
}
return { // public interface
publicMethod1: function () {
// private members can be accessed here
},
publicMethod2: function () {
// ...
}
};
})();
About the new function(){} construct, it will simply use an anonymous function as a constructor function, the context inside that function will be a new object that will be returned.
Edit: In response to the #J5's comment, that is simple to do, actually I think that this can be a nice example for using a Lazy Function Definition pattern:
function singleton() {
var instance = (function() {
var privateVar;
function privateMethod () {
// ...
}
return { // public interface
publicMethod1: function () {
// private members can be accessed here
},
publicMethod2: function () {
// ...
}
};
})();
singleton = function () { // re-define the function for subsequent calls
return instance;
};
return singleton(); // call the new function
}
When the function is called the first time, I make the object instance, and reassign singleton to a new function which has that object instance in it's closure.
Before the end of the first time call I execute the re-defined singleton function that will return the created instance.
Following calls to the singleton function will simply return the instance that is stored in it's closure, because the new function is the one that will be executed.
You can prove that by comparing the object returned:
singleton() == singleton(); // true
The == operator for objects will return true only if the object reference of both operands is the same, it will return false even if the objects are identical but they are two different instances:
({}) == ({}); // false
new Object() == new Object(); // false

I have used the second version (var singleton = {};) for everything from Firefox extensions to websites, and it works really well. One good idea is to not define things inside the curly brackets, but outside it using the name of the object, like so:
var singleton = {};
singleton.dothis = function(){
};
singleton.someVariable = 5;

The ES5 spec lets us use Object.create():
var SingletonClass = (function() {
var instance;
function SingletonClass() {
if (instance == null) {
instance = Object.create(SingletonClass.prototype);
}
return instance;
}
return {
getInstance: function() {
return new SingletonClass();
}
};
})();
var x = SingletonClass.getInstance();
var y = SingletonClass.getInstance();
var z = new x.constructor();
This is nice, since we don't have to worry about our constructor leaking, we still always end up with the same instance.
This structure also has the advantage that our Singleton doesn't construct itself until it is required. Additionally, using the closure as we do here prevents external code from using our "instance" variable, accidentally or otherwise. We can build more private variables in the same place and we can define anything we care to export publically on our class prototype.

The singleton pattern is implemented by creating a class with a method that creates a new instance of the class if one does not exist. If an instance already exists, it simply returns a reference to that object. 1
(function (global) {
var singleton;
function Singleton () {
// singleton does have a constructor that should only be used once
this.foo = "bar";
delete Singleton; // disappear the constructor if you want
}
global.singleton = function () {
return singleton || (singleton = new Singleton());
};
})(window);
var s = singleton();
console.log(s.foo);
var y = singleton();
y.foo = "foo";
console.log(s.foo);
You don't just declare the singleton as an object because that instantiates it, it doesn't declare it. It also doesn't provide a mechanism for code that doesn't know about a previous reference to the singleton to retrieve it. The singleton is not the object/class that is returned by the singleton, it's a structure. This is similar to how closured variables are not closures, the function scope providing the closure is the closure.

I am just posting this answer for people who are looking for a reliable source.
according to patterns.dev by Lydia Hallie, Addy Osmani
Singletons are actually considered an anti-pattern, and can (or.. should) be avoided in JavaScript.
In many programming languages, such as Java or C++, it's not possible to directly create objects the way we can in JavaScript. In those object-oriented programming languages, we need to create a class, which creates an object. That created object has the value of the instance of the class, just like the value of instance in the JavaScript example.
Since we can directly create objects in JavaScript, we can simply use
a regular object to achieve the exact same result.

I've wondered about this too, but just defining an object with functions in it seems reasonable to me. No sense creating a constructor that nobody's ever supposed to call, to create an object with no prototype, when you can just define the object directly.
On the other hand, if you want your singleton to be an instance of some existing "class" -- that is, you want it to have some other object as its prototype -- then you do need to use a constructor function, so that you can set its prototype property before calling it.

The latter code box shows what I've seen JS devs call their version of OO design in Javascript.
Singetons are meant to be singular objects that can't be constructed (except, I suppose, in the initial definition. You have one, global instance of a singleton.

The point of using the "pseudo constructor" is that it creates a new variable scope. You can declare local variables inside the function that are available inside any nested functions but not from the global scope.
There are actually two ways of doing it. You can call the function with new like in your example, or just call the function directly. There are slight differences in how you would write the code, but they are essentially equivalent.
Your second example could be written like this:
var singleton = new function () {
var privateVariable = 42; // This can be accessed by dothis and dothat
this.dothis = function () {
return privateVariable;
};
this.dothat = function () {};
}; // Parentheses are allowed, but not necessary unless you are passing parameters
or
var singleton = (function () {
var privateVariable = 42; // This can be accessed by dothis and dothat
return {
dothis: function () {
return privateVariable;
},
dothat: function () {}
};
})(); // Parentheses are required here since we are calling the function
You could also pass arguments to either function (you would need to add parentheses to the first example).

Crockford (seems to) agree that the object literal is all you need for a singleton in JavaScript:
http://webcache.googleusercontent.com/search?q=cache:-j5RwC92YU8J:www.crockford.com/codecamp/The%2520Good%2520Parts%2520ppt/5%2520functional.ppt+singleton+site:www.crockford.com&cd=1&hl=en&ct=clnk

How about this:
function Singleton() {
// ---------------
// Singleton part.
// ---------------
var _className = null;
var _globalScope = null;
if ( !(this instanceof arguments.callee) ) {
throw new Error("Constructor called as a function.");
}
if ( !(_className = arguments.callee.name) ) {
throw new Error("Unable to determine class name.")
}
_globalScope = (function(){return this;}).call(null);
if ( !_globalScope.singletons ) {
_globalScope.singletons = [];
}
if ( _globalScope.singletons[_className] ) {
return _globalScope.singletons[_className];
} else {
_globalScope.singletons[_className] = this;
}
// ------------
// Normal part.
// ------------
var _x = null;
this.setx = function(val) {
_x = val;
}; // setx()
this.getx = function() {
return _x;
}; // getx()
function _init() {
_x = 0; // Whatever initialisation here.
} // _init()
_init();
} // Singleton()
var p = new Singleton;
var q = new Singleton;
p.setx(15);
q.getx(); // returns 15

I stole this from CMS / CMS' answer, and changed it so it can be invoked as:
MySingleton.getInstance().publicMethod1();
With the slight alternation:
var MySingleton = { // These two lines
getInstance: function() { // These two lines
var instance = (function() {
var privateVar;
function privateMethod () {
// ...
console.log( "b" );
}
return { // public interface
publicMethod1: function () {
// private members can be accessed here
console.log( "a" );
},
publicMethod2: function () {
// ...
privateMethod();
}
};
})();
singleton = function () { // re-define the function for subsequent calls
return instance;
};
return singleton(); // call the new function
}
}