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How to set var self = this; from outside the function?

I have a class-like function
var myapp = function() {
this.method = function() {
//Do something...
}
}
To reference myapp from within methods, the first line in the myapp function is
var self = this;
So a method in myapp can reference the "class" safely
this.anothermethod = function() {
self.method();
}
The full code:
var myapp = function() {
var self = this;
this.dosomething = function(Callback) {
Callback();
}
this.anothermethod = function() {
//Pass a callback ("self" is required here)...
this.dosomething(function() {
self.complete();
)};
}
this.complete = function() {
console.log('All done!');
}
}
My question is: can I assign var self = this; from outside the declaration of myapp? I don't want to set self every single time I write a "class".
Kind of like this:
var library = function() {
this.loadclass = function(Name) {
var tempclass = window[Name];
library[Name] = new tempclass();
library[Name].self = library[Name];
}
}
var myapp = new library();
myapp.loadclass('myapp');
myapp.myapp.dosomething();
It doesn't work as expected. self equals window for some reason.
I know it's a little abnormal programming, but can it be done?
Note about using self: I remember why I started using it. I wanted to reference the base class (this) from within callbacks inside methods. As soon as you try to use this within a function within a method, it then references the method, not the base class.

Unless you are detaching the methods from the object and calling them as plain functions, you don't need a self variable at all. The method can reach its object using the this keyword:
var myapp = function() {
this.method = function() {
//Do something...
}
this.anothermethod = function() {
this.method();
}
}

No, you can't really; not the way you're creating objects at least.
You can sort of do this, by enumerating all the functions on the object and binding them to the object itself. Something like this:
Object.keys(obj)
.filter(function(n) { return typeof obj[n] == "function" })
.forEach(function(n) { obj[n] = obj[n].bind(obj) })
This function will go over the public, enumerable properties of obj and make sure that any functions on it are bound to obj; i.e. this is now bound to obj.
A primer on this
When you call new, this within the constructor gets bound to the newly created object. If you do need a reference to this as it was bound at constructor time, you do need to keep away a reference to it.
Functions in JavaScript are bound to wherever it is called. Here's an example:
var foo = new function() {
this.bar = function() {
return 'bar'
}
this.baz = function() {
return this.bar()
}
}
console.log(foo.bar()) // bar
console.log(foo.baz()) // bar
var bar = function() {
return "window"
}
var baz = foo.baz
console.log(baz()) // window
When we call foo.baz() it'll look to foo for the implementation of bar, but when calling foo.baz through a "detached" reference, it'll look to whatever the global object is (in this case the browser window object) and call bar from there. Because we defined bar in the global context, it then returns window.
The practice of assign a variable called self is so that it doesn't matter how you call your methods, because you always reference the this at the time of creation through the self variable. You don't have to write things this way, but then you should understand that references to this may change under your feet.

Using _self so I always can have object context

What is the proper way to use _self to always have access to an object? Is using _self ok, or is it bad practice?
I want a good way to get at myObject's attributes and methods, even from functions that aren't called in myObject's context. There are solutions like .bind(this), using _self, and jQuery's $.proxy().
For example:
var myObject = {
name: 'Tyrion',
alias: 'imp',
_self: function() {
return this;
},
// I know this context is fine, but let's pretend it's being called from elsewhere.
getAlias: function() {
var _self = myObject._self();
return _self.alias;
}
}

In order to do what you're looking to do, you'd have to change a few things. #elclanrs is right about what your this context is. I'll put two options below.
var myObject = {
name: 'Tyrion',
alias: 'imp',
// I know this context is fine, but let's pretend it's being called from elsewhere.
getAlias: function() {
// you'd have to do this in every method.
var _self = this;
return _self.alias;
}
}
The other option is a bit different, and not as usable, but I'm adding it so you can see it:
var myObject = function() {
var _self = this;
_self.name = 'Tyrion';
_self.alias = 'imp';
_self.getAlias = function() {
return _self.alias;
};
};
in the second instance, getAlias would be better as a prototype method, but you won't have access to the _self variable, only this.

You can do this too but I wouldn't necessarily recommend it.
var obj = {
_self: this.obj, // if you don't have .obj it points to window
thing: 'thingy',
alsoThis: function() {
return 'another thing'
}
};
obj._self;
It's also possible that since it's not within a closure, or a function in otherwords, the context of this._self may be incorrect if the context is changed by the scope which its referenced in.
Generally, I just do var _self = this; right before a function which I am nesting another function within that requires the this context outside the parent function since the nested one will not have the ability to access the value of this being nested.
That's usually not too common in my experience, and you really shouldn't be declaring such a property/var that needs to be used for the purpose that _self vars serve. It isn't a good practice and would be best to not do.
What if you run into a situation where you need to have _self = some other context?

thisis determined by the invocation of the function. (aka, the way the function is called) See my other answers for more details.
var myObject = {
name: 'Tyrion',
alias: 'imp',
_self: function () {
return this;
},
// I know this context is fine, but let's pretend it's being called from elsewhere.
getAlias: function () {
var _self = myObject._self();
return _self.alias;
}
};
//member invocation
console.log(myObject._self() === myObject); // true
var aFucntion = myObject._self;
//functional invocation
console.log(aFucntion() === myObject); // false
console.log(aFucntion() === this); //true
Instead of worrying about the context of this, a workaround is to assign this to a value in an outer function, and then access that value in an inner functions. This is called closure
var MyObject = function (title) {
var _self = this,
helper = function () {
return _self.title + " " + _self.name;
};
this.title = title;
this.fullName = function () {
return helper(); //functional invocation
//if helper used this, this would be global
};
this.name = 'Tyrion';
this.alias = 'imp';
this.getAlias = function () {
//access to _self through closure
return _self.alias;
};
};
//constructor invocation
var aObject = new MyObject("Mr.");
console.log(aObject.getAlias()); //imp
console.log(aObject.fullName()); //Mr. Tyrion
FYI:
If _self returns myObject, context would not mater.
_self: function () {
return myObject;
}

'this' keyword doesn’t always refer to the object on which a method is defined, but instead can change based on specific contexts. How?

I was going through this article http://dev.opera.com/articles/view/objects-in-javascript/
where I read "'this' keyword doesn’t always refer to the object on which a method is defined, but instead can change based on specific contexts."
I couldn't find any example in which 'this' does not refer to the object on a method is defined....please gimme an example if possible

One example that you could realistically run into and not expect this result:
var obj = {
testFunc : function(){
alert("testFunc: " + this);
}
}
obj.testFunc(); // this is the object
setTimeout(obj.testFunc, 1000); // this is window
http://jsfiddle.net/t7ycd/

There are many way to change context. jsfiddle
using bind: (not supported by older IE browsers (IE < 9))
var obj = {};
function fun1(){};
obj2 = {};
obj2.fun1 = fun1.bind(obj);
obj2.fun1(); // context inside fun1 would be obj.
Using apply or call.
var obj = {};
function fun1(){};
obj2 = {};
obj2.fun1 = function(){
fun1.apply(obj, arguments);
//or fun1.call(obj, ar1, arg2,...);
};
obj2.fun1(); // context inside fun1 would be obj.

Here is one way
var a = {};
a.foo = function(){ console.log(this); }
a.foo(); // object a
a.foo.call(window); // window object

If you call a method in class B from class A, the 'this' will refer to the class that is calling the method - A, not the class that it is in.

Here is an example taken directly from the Mozilla documentation
https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Operators/this
var o = {prop: 37};
function independent() {
return this.prop;
}
o.f = independent;
console.log(o.f()); // logs 37
So the function independent is defined in the global context, but is then attached to an object. When it is invoked on that object the context becomes the object on which the function is invoked rather than the object on which the function is defined.
You can achieve similar results using the javascript call and bind methods, and using anonymous functions.

like this
var MyObject = function()
{
this.initialize();
}
MyObject.prototype.SomeMethod = function()
{
//common fix is to use
//var self = this;
(function(){
//this has lost scope
//self will retain the this scope that was desired
})();
}

This pointer from internal function

i have JavaScript components, that has following architecture:
var MyComponent = function(params)
{
setup(params);
this.doSomething()
{
// doing something
};
function setup(params)
{
// Setup
// Interaction logic
var _this = this; // "this" points to DOMWindow, not to created object
$(".some-element").click(function(){
_this.doSomething(); // it craches here, because of above
});
}
};
When something, being controlled by interaction logic, happens, sometimes i must forward execution to "public" methods of component.
In this situation, i have a problem with "this" pointer.
Sample code demonstrates it:
var Item = function()
{
this.say = function()
{
alert("hello");
};
this.sayInternal = function()
{
_sayInternal();
};
function _sayInternal()
{
this.say();
};
};
To test it,
Create an object:
var o = new Item();
This works fine:
o.say(); // alerts "hello"
This crashes:
o.sayInternal();
I get an error:
TypeError: Result of expression 'this.say' [undefined] is not a function.
I think, such a behaviour takes place, because _sayInternal() function is declared (and not assigned to object, like "this.say = function()"). This way, it is shared across all created objects and acts like a static function in C++.
Is this true ?

No, sayInternal is not shared between created objects. But you are right, the created objects don't have access to sayInternal as it is not assigned to them. This function is only local to the constructor function.
this always refers to the context a function is invoked in. If you call it like func(), then this refers to the global object (which is window in browser). If you set the function as property of an object and call it with obj.func(), then this will refer to obj.
If you assign a "bound" function to a variable and call it:
var method = obj.func;
method();
then this will again refer to the global object. In JavaScript, functions are like any other value, they don't have a special relationship to the object they are assigned to.
You can explicitly set the context with call or apply:
var MyComponent = function(params)
{
setup.call(this, params); // <- using `call`
this.doSomething()
{
// doing something
};
function setup(params)
{
// Setup
// Interaction logic
var _this = this; // "this" to new created object
$(".some-element").click(function(){
_this.doSomething();
});
}
};
or in you other example:
var Item = function()
{
this.say = function()
{
alert("hello");
};
this.sayInternal = function()
{
_sayInternal.call(this);
};
function _sayInternal()
{
this.say();
};
};
That said, this approach to assign functions to objects is not good, because every instance will have its own this.sayInternal function. So for the Item code above, every creation of an instance involves creating three functions too, which is a waste of memory.
Making use of prototype inheritance would be a better way:
var Item = function() {
};
Item.prototype = (function() {
function _sayInternal() {
this.say();
};
return {
say: function() {
alert("hello");
},
sayInternal: function(){
_sayInternal.call(this);
}
}
}());
This way, _sayInternal is only created once and all instances inherit (refer to) the prototype, so say and sayInternal also exist only once. The "trick" with the immediate function makes _sayInternal only accessible by say and sayInternal.

Global instance in prototypal javascript

I am exploring a pattern where you can save a global reference to the current instance for simple use in anonymous functions where the this keyword is out of context. Consider this:
var instance;
var a = function(id) {
instance = this;
this.id = id;
};
a.prototype.get = function() {
return (function() {
return instance.id;
})();
};
var x = new a('x');
var y = new a('y');
console.log(x.get());
console.log(y.get());
This obviously won't work since instance is defined in the constructor, to each time .get() is called, instance will reference the last constructed object. So it yields 'y' both times.
However, I'm looking for a way to grab the instance inside a prototype method without using the this keyword, to make the code more readable. Singletons is not an option here, I need the prototypal inheritance.

Edit: Since you're avoiding to store "local instances", there are some approaches, for example:
Using call or apply to change the this value of the invoked function:
var a = function (id) {
this.id = id;
};
a.prototype.get = function () {
return (function() {
return this.id; // the inner `this` value will refer to the value of outside
}).call(this);
};
Using an argument:
//..
a.prototype.get = function () {
return (function(instance) {
return instance.id;
})(this);
};
The new ECMAScript 5th Edition introduced the bind method, which is extremely useful to persist the this value and optional bound arguments, you can find a compatible implementation here:
//..
a.prototype.get = function() {
var boundFunction = (function () {
return this.id;
}).bind(this);
return boundFunction(); // no matter how the function is invoked its `this`
}; // value will always refer to the instance.