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How can I pass 'this' object to lower function in javascript? [duplicate]

In C++, the language I'm most comfortable with, usually one declares an object like this:
class foo
{
public:
int bar;
int getBar() { return bar; }
}
Calling getBar() works fine (ignoring the fact that bar might be uninitialized). The variable bar within getBar() is in the scope of class foo, so I don't need to say this->bar unless I really need to make it clear that I'm referring to the class' bar instead of, say, a parameter.
Now, I'm trying to get started with OOP in Javascript. So, I look up how to define classes and try the same sort of thing:
function foo()
{
this.bar = 0;
this.getBar = function() { return bar; }
}
And it gives me bar is undefined. Changing the bar to this.bar fixes the issue, but doing that for every variable clutters up my code quite a bit. Is this necessary for every variable? Since I can't find any questions relating to this, it makes me feel like I'm doing something fundamentally wrong.
EDIT: Right, so, from the comments what I'm getting is that this.bar, a property of an object, references something different than bar, a local variable. Can someone say why exactly this is, in terms of scoping and objects, and if there's another way to define an object where this isn't necessary?

JavaScript has no classes class-based object model. It uses the mightier prototypical inheritance, which can mimic classes, but is not suited well for it. Everything is an object, and objects [can] inherit from other objects.
A constructor is just a function that assigns properties to newly created objects. The object (created by a call with the new keyword) can be referenced trough the this keyword (which is local to the function).
A method also is just a function which is called on an object - again with this pointing to the object. At least when that function is invoked as a property of the object, using a member operator (dot, brackets). This causes lots of confusion to newbies, because if you pass around that function (e.g. to an event listener) it is "detached" from the object it was accessed on.
Now where is the inheritance? Instances of a "class" inherit from the same prototype object. Methods are defined as function properties on that object (instead of one function for each instance), the instance on which you call them just inherits that property.
Example:
function Foo() {
this.bar = "foo"; // creating a property on the instance
}
Foo.prototype.foo = 0; // of course you also can define other values to inherit
Foo.prototype.getBar = function() {
// quite useless
return this.bar;
}
var foo = new Foo; // creates an object which inherits from Foo.prototype,
// applies the Foo constructor on it and assigns it to the var
foo.getBar(); // "foo" - the inherited function is applied on the object and
// returns its "bar" property
foo.bar; // "foo" - we could have done this easier.
foo[foo.bar]; // 0 - access the "foo" property, which is inherited
foo.foo = 1; // and now overwrite it by creating an own property of foo
foo[foo.getBar()]; // 1 - gets the overwritten property value. Notice that
(new Foo).foo; // is still 0
So, we did only use properties of that object and are happy with it. But all of them are "public", and can be overwritten/changed/deleted! If that doesn't matter you, you're lucky. You can indicate "privateness" of properties by prefixing their names with underscores, but that's only a hint to other developers and may not be obeyed (especially in error).
So, clever minds have found a solution that uses the constructor function as a closure, allowing the creating of private "attributes". Every execution of a javascript function creates a new variable environment for local variables, which may get garbage collected once the execution has finished. Every function that is declared inside that scope also has access to these variables, and as long as those functions could be called (e.g. by an event listener) the environment must persist. So, by exporting locally defined functions from your constructor you preserve that variable environment with local variables that can only be accessed by these functions.
Let's see it in action:
function Foo() {
var bar = "foo"; // a local variable
this.getBar = function getter() {
return bar; // accesses the local variable
}; // the assignment to a property makes it available to outside
}
var foo = new Foo; // an object with one method, inheriting from a [currently] empty prototype
foo.getBar(); // "foo" - receives us the value of the "bar" variable in the constructor
This getter function, which is defined inside the constructor, is now called a "privileged method" as it has access to the "private" (local) "attributes" (variables). The value of bar will never change. You also could declare a setter function for it, of course, and with that you might add some validation etc.
Notice that the methods on the prototype object do not have access to the local variables of the constructor, yet they might use the privileged methods. Let's add one:
Foo.prototype.getFooBar = function() {
return this.getBar() + "bar"; // access the "getBar" function on "this" instance
}
// the inheritance is dynamic, so we can use it on our existing foo object
foo.getFooBar(); // "foobar" - concatenated the "bar" value with a custom suffix
So, you can combine both approaches. Notice that the privileged methods need more memory, as you create distinct function objects with different scope chains (yet the same code). If you are going to create incredibly huge amounts of instances, you should define methods only on the prototype.
It gets even a little more complicated when you are setting up inheritance from one "class" to another - basically you have to make the child prototype object inherit from the parent one, and apply the parent constructor on child instances to create the "private attributes". Have a look at Correct javascript inheritance, Private variables in inherited prototypes, Define Private field Members and Inheritance in JAVASCRIPT module pattern and How to implement inheritance in JS Revealing prototype pattern?

Explicitly saying this.foo means (as you've understood well) that you're interested about the property foo of the current object referenced by this. So if you use: this.foo = 'bar'; you're going to set the property foo of the current object referenced by this equals to bar.
The this keyword in JavaScript doesn't always mean the same thing like in C++. Here I can give you an example:
function Person(name) {
this.name = name;
console.log(this); //Developer {language: "js", name: "foo"} if called by Developer
}
function Developer(name, language) {
this.language = language;
Person.call(this, name);
}
var dev = new Developer('foo', 'js');
In the example above we're calling the function Person with the context of the function Developer so this is referencing to the object which will be created by Developer. As you might see from the console.log result this is comes from Developer. With the first argument of the method call we specify the context with which the function will be called.
If you don't use this simply the property you've created will be a local variable. As you might know JavaScript have functional scope so that's why the variable will be local, visible only for the function where it's declared (and of course all it's child functions which are declared inside the parent). Here is an example:
function foo() {
var bar = 'foobar';
this.getBar = function () {
return bar;
}
}
var f = new foo();
console.log(f.getBar()); //'foobar'
This is true when you use the var keyword. This means that you're defining bar as local variable if you forget var unfortunately bar will became global.
function foo() {
bar = 'foobar';
this.getBar = function () {
return bar;
}
}
var f = new foo();
console.log(window.bar); //'foobar'
Exactly the local scope can help you to achieve privacy and encapsulation which are one of the greatest benefits of OOP.
Real world example:
function ShoppingCart() {
var items = [];
this.getPrice = function () {
var total = 0;
for (var i = 0; i < items.length; i += 1) {
total += items[i].price;
}
return total;
}
this.addItem = function (item) {
items.push(item);
}
this.checkOut = function () {
var serializedItems = JSON.strigify(items);
//send request to the server...
}
}
var cart = new ShoppingCart();
cart.addItem({ price: 10, type: 'T-shirt' });
cart.addItem({ price: 20, type: 'Pants' });
console.log(cart.getPrice()); //30
One more example of the benefits of the JavaScript scope is the Module Pattern.
In Module Pattern you can simulate privacy using the local functional scope of JavaScript. With this approach you can have both private properties and methods. Here is an example:
var module = (function {
var privateProperty = 42;
function privateMethod() {
console.log('I\'m private');
}
return {
publicMethod: function () {
console.log('I\'m public!');
console.log('I\'ll call a private method!');
privateMethod();
},
publicProperty: 1.68,
getPrivateProperty: function () {
return privateProperty;
},
usePublicProperty: function () {
console.log('I\'ll get a public property...' + this.publicProperty);
}
}
}());
module.privateMethod(); //TypeError
module.publicProperty(); //1.68
module.usePublicProperty(); //I'll get a public property...1.68
module.getPrivateProperty(); //42
module.publicMethod();
/*
* I'm public!
* I'll call a private method!
* I'm private
*/
There's a little strange syntax with the parentless wrapping the anonymous functions but forget it for the moment (it's just executing the function after it's being initialized). The functionality can be saw from the example of usage but the benefits are connected mainly of providing a simple public interface which does not engages you with all implementation details. For more detailed explanation of the pattern you can see the link I've put above.
I hope that with this :-) information I helped you to understand few basic topics of JavaScript.

function Foo() {
this.bar = 0;
this.getBar = function () { return this.bar };
}
When you call the function above with the new keyword - like this...
var foo = new Foo();
... - a few things happen:
1) an object is created
2) the function is executed with the this keyword referencing that object.
3) that object is returned.
foo, then, becomes this object:
{
bar: 0,
getBar: function () { return this.bar; }
};
Why not, then, just do this:
var foo = {
bar: 0,
getBar: function () { return this.bar; }
};
You would, if it's just that one simple object.
But creating an object with a constructor (that's how it's called) gives us a big advantage in creating multiple of the "same" objects.
See, in javascript, all functions are created with a prototype property [an object], and all objects created with that function (by calling it with the new keyword) are linked to that prototype object. This is why it's so cool - you can store all common methods (and properties, if you wanted to) in the prototype object, and save a lot of memory. This is how it works:
function Foo( bar, bob ) {
this.bar = bar;
this.bob = bob;
}
Foo.prototype.calculate = function () {
// 'this' points not to the 'prototype' object
// as you could've expect, but to the objects
// created by calling Foo with the new keyword.
// This is what makes it work.
return this.bar - this.bob;
};
var foo1 = new Foo(9, 5);
var foo2 = new Foo(13, 3);
var result1 = foo1.calculate();
var result2 = foo2.calculate();
console.log(result1); //logs 4
console.log(result2); //logs 10
That's it!

To get closer to OOP in JavaScript, you might want to take a look into a Module design pattern (for instance, described here).
Based on the closure effect, this pattern allows emulating private properties in your objects.
With 'private' properties you can reference them directly by its identifier (i.e., no this keyword as in constructors).
But anyway, closures and design patterns in JS - an advanced topic. So, get familiar with basics (also explained in the book mentioned before).

In javascript this always refers to the owner object of the function. For example, if you define your function foo() in a page, then owner is the javascript object windows; or if you define the foo() on html element <body>, then the owner is the html element body; and likewise if you define the function onclick of element <a>, then the owner is the anchor.
In your case, you are assigning a property bar to the 'owner' object at the begining and trying to return the local variable bar.
Since you never defined any local varialbe bar, it is giving you as bar is undefined.
Ideally your code should have defined the variable as var bar; if you want to return the value zero.

this is like a public access modifier of objects(variables or functions), while var is the private access modifier
Example
var x = {};
x.hello = function(){
var k = 'Hello World';
this.m = 'Hello JavaScript';
}
var t = new x.hello();
console.log(t.k); //undefined
console.log(t.m); //Hello JavaScript

Wrong bind techniques?

Did you ever think about what exactly bind does and if bind can be overused?
This is model of the object that is considered as a bad practise (example A):
var A = function() {
this.foo = "foo";
this.bar = function () {
console.log("bar");
};
};
This is same object written better:
var A = function() {
this.foo = "foo";
};
A.prototype.bar = function () {
console.log("bar");
};
Function bar is not declared over and over for each instance of A. Now look at the following snippet:
function origin() {
console.log(this.toString() + " is who I am");
};
var listOfBindedF = [];
var dummyObject = {};
for(var i = 0; i<100;i++) {
listOfBindedF.push(origin.bind(dummyObject));
}
With each bind usage, I declared new function didnt I? A lot of libs, frameworks and engines uses bind in its core. Not just for async calls, where it is really hard to go without bind, but literally everywhere, custom inheritance is good example.
I did short test to check it on my own.
Array with 100k length and full of references to origin eats
~465kB.
Array with 200k length, 100k references to origin and 100k
references to dummyObject eats ~1.1MB.
Array with 100k length full
of origin.bind(dummyObject) eats ~9.5MB.
So is there any bind performance impact on javascript apps we are doing with these frameworks/libs? Is the impact noticable? Isnt it similar to example A?

First off, this is a function declaration
function foo() {
}
and this is a function expression
var foo = function() {};
Your two As are different. Imagine the following code:
function A(foo) {
this.bar = function() {
console.log(foo);
};
}
This keeps foo private and still allows bar to access it, something not possible with the prototype version of A.
As for the memory issue: Each function object takes up some memory, obviously. A function expression always creates a new object. Creating 100 instances of A would create 100 function objects assigned to bar using your first example.
A function declaration creates an instance when its surrounding scope is entered. So 100 instances of A would still create 100 instances of bar in this case:
function A(foo) {
function bar() {
console.log(foo);
}
this.bar = bar;
}
bind also creates a new function object that needs even more memory (it has to store a reference to the original function, the value for this etc.)
Using bind has an impact, but I doubt that that will ever lead to performance problems in your app. FWIW: Calling bar in your first example for A is faster then calling it in your second one, because it's not necessary to go up the prototype chain.
Please note that bind can be used for currying as well, making it a versatile tool:
function log(message) {
console.log(message);
}
var logHello = log.bind(null, 'Hello');
logHello(); // prints 'Hello' to the console
Conclusion: Use the tool that's best suited for a task.

Javascript: why "this" inside the private function refers to the global scope?

Consider the following code:
function A() {}
A.prototype.go = function() {
console.log(this); //A { go=function()}
var f = function() {
console.log(this); //Window
};
f();
}
var a = new A();
a.go();
Why does 'this' inside function 'f' refers to the global scope? Why it is not the scope of function 'A' ?

JavaScript has a different concept of what the special name this refers to
than most other programming languages do. There are exactly five different
ways in which the value of this can be bound in the language.
The Global Scope
this;
When using this in global scope, it will simply refer to the global object.
Calling a Function
foo();
Here, this will again refer to the global object.
ES5 Note: In strict mode, the global case no longer exists.
this will instead have the value of undefined in that case.
Calling a Method
test.foo();
In this example, this will refer to test.
Calling a Constructor
new foo();
A function call that is preceded by the new keyword acts as
a constructor. Inside the function, this will refer
to a newly created Object.
Explicit Setting of this
function foo(a, b, c) {}
var bar = {};
foo.apply(bar, [1, 2, 3]); // array will expand to the below
foo.call(bar, 1, 2, 3); // results in a = 1, b = 2, c = 3
When using the call or apply methods of Function.prototype, the value of
this inside the called function gets explicitly set to the first argument
of the corresponding function call.
As a result, in the above example the method case does not apply, and this
inside of foo will be set to bar.
Note: this cannot be used to refer to the object inside of an Object
literal. So var obj = {me: this} will not result in me referring to
obj, since this only gets bound by one of the five listed cases.
Common Pitfalls
While most of these cases make sense, the first one is to be considered another
mis-design of the language because it never has any practical use.
Foo.method = function() {
function test() {
// this is set to the global object
}
test();
}
A common misconception is that this inside of test refers to Foo; while in
fact, it does not.
In order to gain access to Foo from within test, it is necessary to create a
local variable inside of method which refers to Foo.
Foo.method = function() {
var that = this;
function test() {
// Use that instead of this here
}
test();
}
that is just a normal variable name, but it is commonly used for the reference to an
outer this. In combination with closures, it can also
be used to pass this values around.
Assigning Methods
Another thing that does not work in JavaScript is function aliasing, which is
assigning a method to a variable.
var test = someObject.methodTest;
test();
Due to the first case, test now acts like a plain function call; therefore,
this inside it will no longer refer to someObject.
While the late binding of this might seem like a bad idea at first, in
fact, it is what makes prototypal inheritance work.
function Foo() {}
Foo.prototype.method = function() {};
function Bar() {}
Bar.prototype = Foo.prototype;
new Bar().method();
When method gets called on a instance of Bar, this will now refer to that
very instance.
Disclaimer: Shamelessy stolen from my own resources at http://bonsaiden.github.com/JavaScript-Garden/#function.this

The reason why is you are invoking f as a function and not a method. When invoked as a function this is set to window during the execution of the target
// Method invocation. Invoking a member (go) of an object (a). Hence
// inside "go" this === a
a.go();
// Function invocation. Invoking a function directly and not as a member
// of an object. Hence inside "f" this === window
f();
// Function invocation.
var example = a.go;
example();

The scope of all functions is window.
To circumvent that, you can do this:
function A() {}
A.prototype.go = function() {
var self = this;
console.log(self); //A { go=function()}
var f = function() {
console.log(self); //A { go=function()}
};
f();
}

Because function f() is not called without any object reference. Try,
f.apply(this);

JavaScript library pattern

I'm trying to figure out the basic pattern for creating a JavaScript library (class). I want to do it in such a way that it doesn't pollute the global namespace with a bunch of junk, but allowing for the class to have instance variables and public methods that modify those instance variables.
Consider the following toy example. I want to make a class Foo. It should contain an instance member, bar, which is a number. There should be a constructor for Foo that takes a number and initializes its instance bar with that number. There should be an instance method that I can call on a Foo object to modify bar. Maybe the code that uses the library looks like this:
var foo1 = new Foo(1);
var foo2 = new Foo(2);
console.log(foo1.bar); // should print "1"
console.log(foo2.bar); // should print "2"
foo2.changeBar(42);
console.log(foo1.bar); // should print "1"
console.log(foo2.bar); // should print "42"
The resultant foo.js would be used by a Web app and therefore included via a script tag in the HTML.
I've done a bit of searching with Google but I have yet to find a single, concise, generic outline of how to design a JavaScript class (used as a library).

(function () {
Foo = function (num) {
this.changeBar(num);
};
var privateMethod = function (x) {
if (this.bar === 999) {
this.bar = x;
}
};
Foo.prototype.changeBar = function (num) {
this.bar = num;
privateMethod.call(this, 1);
};
}());
That is the simplest way of doing it. You don't need to include the definition in a closure, more of a style thing.

Building up on Evan's answer, to showcase some more possibilites. Most normal cases only use some of these though.
(function() {
//When we create variables inside a function they can only be seen by other
// inner functions. Wraping all our code here makes sure noone gets to see
// the private stuff.
//The first opening parenthesis is required for Javascript to parse it
//correctly though
//this is the constructor function
//Note how we set a global variable (Foo) to make it available outside.
Foo = function(num, another_num) {
this.changeBar(num);
//sometimes you will want to make a method that modifies a variable
//that can't be acessed via this.xxx. You can use a closure here for that
//(there are some performance considerations though)
this.baz = function(){
console.log(another_num);
}
//Setting methods "by hand" like this is also useful if you want to
//do multiple inheritance, since you can just set all methods of the
//second class by hand here if you want.
}
//Things on Foo.prototype will be available for all Foo objects,
// via prototypal inheritance magic.
Foo.prototype.changeBar = function(num) {
this.bar = num;
}
var a_secret_variable = 42;
function my_private_function(){
console.log(a_secret_variable);
}
//All private variables can be normaly used (by functions that can see them).
Foo.prototype.use_magic = function(){
my_private_function();
}
}());
//The "fake" function is imediatelly called,
//so in the end all it does is create a inner scope.

The Module Pattern is probably the most popular pattern used today.
var Foo = (function() {
var _thisIsAPrivateVar;
function thisIsAPrivateMethod() {
}
return {
thisIsAPublicMethod : function() {
// can still access the "private" variable and method
}
};
})();

Can someone explain this OOP javascript structure

Can someone explain this OOP javascript structure?
I realize it is how you create 'objects' in javascript, just need some explanation on the notation and what it means:
var vote = function(){
return {
P1: function() {
alert('P1');
},
P2: function() {
alert('P2');
}
};
}();
vote.P1();
vote.P2();
Why are the public methods in the return call? How is this possible? Separated by commas?
Why does the end of the 'object' have to be (); ?
Naming convention for internal methods and public methods?
Are public properties and methods the same structure? What is the difference?

var vote = function(){
var privateMember = 3; // lets define a private member, this will only be available inside the function scope, so we can't do "vote.privateMember" because we're not returning it below
// It's returning an object, {}
// these are considered public because they are returned by the function, making them available outside the function scope using the "dot" operator, "vote.P1()"
return {
// first value of the object is a function
// so we can call this like "vote.P1()"
// we're calling a privateMember in here
P1: function() {
alert('P1');
alert(privateMember);
},
// second value of the object is a function
// so we can call this like "vote.P2()"
P2: function() {
alert('P2');
}
};
}(); // () means it will execute the function, so "vote" will store the return value of this function, it's a shorthand for `var vote = function(){}; vote = vote();
Naming convention for private methods is usually to put an underscore before the method/property name _privateMethod: function(){}.

It's returning the object/hash. Notice that right after the return is a { which is the beginning of the object/hash.
to run the anonymous function that returns the object.
I don't know what you mean.
Yes they are the same. A property can be a function because javascript has first class functions.
This code is contrived so it's hard to say anything useful about it. I'm not sure what the authors intent was. It looks like he may have been aiming to create a class like thing, as opposed to the way javascript does prototype based OO. This code could have just as easily been written.
var vote = {
P1: function() {
alert('P1');
},
P2: function() {
alert('P2');
}
};

Three important concepts: anonymous functions, object literals, closure.
Anonymous Functions
You can declare and execute a function without assigning it to a variable. In you example, foo isn't a function, it's the result of calling:
var item = function(){ /*function code here*/ }**()**; <- Because the function is being executed, item is the object the function returned, not the function.
Object Literals
Say you did this: var a = new Object(); a.foo = 'bar';
Object literal notation would look like this: var a = {foo: 'bar'};
In you example, your anonymous function is returning a new object with two functions, P1 and P2. It's just a different notation for specifying objects.
Closure
When a function returns an object, that object retains access to the functions scope - this is called a closure. So, if you do this:
var a = function(){
var _pvt = 'foo';
return {
aFunc: function(){alert(_pvt);}
};
}();
a.aFunc();
_pvt's scope is the anonymous function. This function returns a new object with one method (aFunc). The anonymous function's scope is available to this object through closure, so when a.aFunc() is called, _pvt will be available.
Best place to learn about this is the Douglas Crockford videos on YUI Theater: http://video.yahoo.com/watch/111585/1027823

This is JSON (javascript object notations). It's similar to dictionary format in python.
Arrays can be defined inline using
[1,2,3,4,5]
and objects can be defined inline using
{ field1: value1, field2: value2, field3: value3 }
A function can be defined inline too,
var a = function() { .... } //a is a function
The () after the function is used to call the function immediately after its definition! Because the function is defined inline, it's like saying:
x = function() { ... }
y = x();
but condensed as:
y = function(){...} ();
As far as I know, there's no public or private, it's all public, and functions are no different from variables, it's just that their type is a function.