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difference between Prototype constructor and private property

Consider the first scenario:
function f(){
console.log("inside the function : " + f.myVar);
}
f.prototype.myVar = 1;
var myObject = new f();
console.log("from the object : " + myObject.myVar);
And this is the output:
inside the function : undefined
from the object : 1
Question: why myVar isn't available in function? If it is stored in the objects prototype it is supposed to be accessible in f().
Now this scenario:
function f(){
console.log("inside the function : " + this.myVar);
}
f.prototype.myVar = 1;
var myObject = new f();
console.log("from the object : " + myObject.myVar);
And the output:
inside the function : 1
from the object : 1
Question: Why I'm getting different result? if 'this' refers to the object doesn't f.myVar mean access myVar in myObject ?
And now this scenario:
function f(){
console.log("inside the function : " + f.myVar);
console.log("inside the function using 'this' : " + this.myVar);
}
f.myVar = 1;
var myObject = new f();
console.log("from the object : " + myObject.myVar);
output:
inside the function : 1
inside the function using 'this' : undefined
from the object : undefined
So if I set a property without using prototype it should'nt be available in instantiated objects. But if I write the script like this, it'll deliver a strange result:
function f(){
console.log("inside the function : " + f.myVar);
}
f.myVar = 1;
var myObject = new f();
console.log("from the first object : " + myObject.myVar);
var strangeObject = myObject.constructor;
console.log("from the strange object : " + strangeObject.myVar);
Output:
inside the function : 1
from the first object : undefined
from the strange object : 1
Where does that "f.myVar" store? Whose variable is that? I'm confused what is the difference between all scenarios above. A complete carification would be much appreciated.
EDIT:
The main problem is that I don't know what this exactly mean:
function f(){}
f.someVar = someValue;
Because in other languages function is an abstract concept and in fact doesn't exist until it is called. Now in JS it is said that functions are objects by default. Ok so I should have an object like this by the script above:
{someVar : sameValue}
in fact I think this should be as same as:
function f(){this.someVar = someValue;} //should create {someVar : someValue}
If this is the case EVERY object instantiated by calling "new f()" must contain this "someVar" but they don't.

Start out by examining the definition of the word prototype. I think it's important to keep this in mind when thinking about how new objects are created in JavaScript.
pro·to·type
noun
a first, typical or preliminary model of something, especially a machine, from which other forms are developed or copied.
verb
make a prototype of (a product).
A prototype is a model from which another form will be copied.
When you create a new object in JavaScript, that is exactly what happens.
var obj = new MyObject();
In the above code, there are many things that happen, but in the context of the question, there are two things that happen which are relevant:
The prototype is applied to a new object.
The function MyObject is called, with this set to the new object.
With this knowledge in mind, let's take a look at the different forms of setting variables you have described:
function MyObject() {}
MyObject.myProperty = 'MyProperty';
It's important to understand that functions themselves are objects in JavaScript. Therefore, function MyObject is an object instance to itself. On the second line, we have set the property myProperty on this function object.
Refer back to the steps of creation above, and you will notice that it does not include applying properties from the function object to the new instance object. It only applies properties from the function object's prototype, and then runs the body of the function with this set to the new instance.
function MyObject() {
this.myProperty = 'MyProperty';
}
Here, the property myProperty is set on the individual instance.
function MyObject() {}
MyObject.prototype.myProperty = 'MyProperty';
In this example, each new instance of MyObject will be given its own property called myProperty and the value set to 'MyProperty'. From there, each instance can change its own myProperty to whatever value it needs without affecting the other.
function MyObject() {
console.log('myProperty', this.myProperty); //Will output 'Outside constructor.'
this.myProperty = 'Inside constructor.';
console.log('myProperty', this.myProperty); //Will output 'Inside constructor.
}
MyObject.prototype.myProperty = 'Outside constructor.';
The above example shows how myProperty is first applied from the prototype, then overwritten by the value applies in the function that is run.
Let's take a look at an example with all the forms you mentioned:
var output1 = document.getElementById('output1'),
output2 = document.getElementById('output2'),
output3 = document.getElementById('output3');
function MyObject(myProperty) {
this.myProperty = myProperty;
}
MyObject.myProperty = 'Function property.';
MyObject.prototype.myProperty = 'Prototype property.';
var obj = new MyObject('Constructor property');
output1.innerHTML = obj.myProperty;
output2.innerHTML = MyObject.myProperty;
output3.innerHTML = MyObject.prototype.myProperty;
<div id="output1"></div>
<div id="output2"></div>
<div id="output3"></div>
In the above example, you'll see how each can be referenced. Now examine it a little closer. Take a look at what happens with your 'Function property' when it is set from two different object instances:
var output1 = document.getElementById('output1'),
output2 = document.getElementById('output2');
function MyObject() {
//We are concatenating a string to the end of the property on each function call.
MyObject.myProperty += ' test ';
}
MyObject.myProperty = 'Function property.';
var obj1 = new MyObject();
var obj2 = new MyObject();
output1.innerHTML = MyObject.myProperty;
output2.innerHTML = MyObject.myProperty;
<div id="output1"></div>
<div id="output2"></div>
The above code demonstrates how the function level property is effectively shared. That's because it's not part of each instance. It's part of the function object.
Here I'll show you the process that takes place with the new operator, without actually using the new operator:
var output = document.getElementById('output');
//Let's have an object that has a prototype property with some properties:
var MyPrototypeObject = {
prototype: {
myPrototypeProperty: 'Prototype property'
}
};
//Let's specify a function that will be used as a constructor:
function MyConstructorFunction() {
this.myInstanceProperty = 'Instance property';
}
//First, new creates an object
var obj = {};
//Next, it applies all the properties from the prototype. We are using the MyPrototypeObject's prototype property for this example
for (var key in MyPrototypeObject.prototype) {
var val = MyPrototypeObject.prototype[key];
//Set the same property on the new object.
obj[key] = val;
}
//Now the prototype has been applied, let's apply the constructor function that was called.
MyConstructorFunction.call(obj); //This calls MyConstructorFunction with this set to obj.
output.innerHTML = 'myPrototypeProperty: ' + obj.myPrototypeProperty + '<br>' + 'myInstanceProperty: ' + obj.myInstanceProperty;
<div id="output"></div>

why myVar isn't available in function? If it is stored in the objects
prototype it is supposed to be accessible in f().
It's accessible in the function, but not as f.myVar, but as this.myVar or f.prototype.myVar.
Why I'm getting different result? if 'this' refers to the object
doesn't f.myVar mean access myVar in myObject ?
The function f is not the same as the object instance. The function is the constructor for the object, and using it with the new keyword creates an instance that is a separate object from the function.
When you use f.var, that is the property var of the function object. When you use this.var in the function, that is the property var in the object instance that the use of the new keyword created.
If you use f.var, that is a property of the constructor function object, so it will be the same variable even if you create multiple instances of the object, and it's only accessible using f.var.
If you use f.prototype.var, that will also be a variable that is the same for all instances of the object, but that can also be accessed using this.var as the object inherits the members of the prototype.
Example:
function f() {
console.log(f.var); // shows "42"
console.log(f.prototype.var); // shows "0.01"
console.log(this.var); shows "0.01";
}
f.var = 42;
f.prototype.var = 0.01;
If you want a variable that is local to each instance of the object, you should use neither of those. You should assign a value to this.var, which will make it a property in the object instance.
Example:
function f(value) {
this.var = value;
}
f.prototype.getValue = function(){
return this.var;
};
var instance1 = new f(42);
var instance2 = new f(0.01);
// now you have two instances with separate values:
console.log(instance1.getValue()); // shows "42"
console.log(instance2.getValue()); // shows "0.01"

As for question in edit, really all simple :-)
you have a function object function f(){}
you add to this object property, like you can add property to any other object in javascript f.someVar = 1
this not same as function f(){this.someVar = someValue;} because this in javascript depends on how function calling and can refers to created object, global object, or something else if called with call or apply function.
When you create object with new operator - you call f as constructor, and in this case this inside function referes to created object, and all properties added inside function as this.something = val add to created object.
Note: that you not use any property from function directly, so this not added to created object.
As for prototype: when you create object - you just set prototype property of created object to f.prototype object. So at end when you create object, you not use any property directly added to function object, just property from prototype and property that added manually to this in constructor

Seems you are getting confused with the prototype of function and object.
Here's a quote from the book Eloquent Javascript explaining the difference:
It is important to note the distinction between the way a prototype is
associated with a constructor (through its prototype property) and the
way objects have a prototype (which can be retrieved with
Object.getPrototypeOf). The actual prototype of a constructor is
Function.prototype since constructors are functions. Its prototype
property will be the prototype of instances created through it but is
not its own prototype.

Javascript: Passing constructor as reference in a variable and calling this constructor

I need to store the reference to a constructor of a class in a variable and construct the object later. This is my minimal code example:
function A() {
var _foo = "A";
}
function Wrapper( constructorFunc ) {
var _constructorFunc = constructorFunc;
this.constructorFunc = function() {
return _constructorFunc;
}
}
var wrapper = new Wrapper( A.constructor );
var cFunc = wrapper.constructorFunc();
var obj = new cFunc(); /* obj should be an A now */
I hope it is clear what I would like to do. The Firebug console gives the error TypeError: cFunc is not a constructor. What is the correct way?
Moreover I must be able to "compare" constructors, i.e. I need to know if two references point to the same constructor. (In C++ this would be possible, because one compares the function's address.) As an example:
function A() {
var _foo = "A";
}
function B() {
var _bar = "B";
}
function Wrapper( constructorFunc ) {
var _constructorFunc = constructorFunc;
this.constructorFunc = function() {
return _constructorFunc;
}
}
var wrapper1 = new Wrapper( A.constructor );
var wrapper2 = new Wrapper( A.constructor );
var wrapper3 = new Wrapper( B.constructor );
wrapper1.constructorFunc() == wrapper2.constructorFunc() /* should evaluate to true */
wrapper1.constructorFunc() == wrapper3.constructorFunc() /* should evaluate to false */
Is this possible?

The constructor is A, so you must change
var wrapper = new Wrapper( A.constructor );
to
var wrapper = new Wrapper( A );
You can test the result with
console.log(obj instanceof A);
For your second question : there's nothing preventing you to compare constructors (or any function) : two functions are equal when they're the same.

In JavaScript, any function can be a constructor, but function's don't have a constructor property.
Therefore, A.constructor should be changed to just A.
However, functions do have prototypes, and the constructor property of their prototypes refers back to them. Therefore, A.prototype.constructor would work as well, but would be a rather wierd way to refer to A.
If your Wrapper only encapsulates a constructor to return it later it seems that you introduced an unecessary object. Just pass the constructor function around directly.
Finally, you can use the instanceof operator to know if an object is an instance of a specific constructor. The advantage of instanceof over comparing constructor functions together is that the inheritance hierarchy will be considered.

Difference between methods of defining JavaScript 'classes'

What's the difference between these two method of defining a 'class' in JavaScript?
Method One
Define method within the constructor:
function MyClass()
{
this.foo = function() { console.log('hello world!'); };
}
Method Two
Define method on the prototype:
function MyClass()
{}
MyClass.prototype.foo = function() { console.log('hello world!'); };

The first will create a new function object on each instantiation of your object, the second will assign a reference to a prototype method to each instance. In short: the second is more efficient, because all instances will share a single function object.
That's just the logic of a prototype chain, you can try and access anything via any object:
var objLiteral = {foo:'bar'};
When accessing objLiteral.foo JS will first look at the properties that the object itself has defined, and return the value if it is found. If JS can't find the property on the object itself, it'll check the object's prototype, hence:
objLiteral.valueOf();//method defined #Object.prototype
objLiteral.valueOf === Object.prototype.valueOf //true
But when you use your first method:
function SomeConstructor()
{
this.methd = function()
{
return true;
}
}
var f = new SomeConstructor();
var g = new SomeConstructor();
f.methd === g.methd;//FALSE!
That shows that we're dealing with 2 separate function objects. Move the function definition to the prototype and f.methd === g.methd; will be true:
function SomeConstructor()
{
}
SomeConstructor.prototype.methd = function()
{
return true;
}
var f = new SomeConstructor();
var g = new SomeConstructor();
f.methd === g.methd;//true!
In response to your comment:
Defining a method on a prototype-level allows you to change a method for a specific task, and then "reset" it back to it's default behaviour. Suppose you're in a function that's creating an AJAX request:
someObject.toString = function(){ return JSON.stringify(this);}
//when concatinating this object you'll get its json string
//do a lot of stuff
delete (someObject.toString);
Again JS will check if the object has the toString property defined on itself, which it has. So JS will delete the function you've assigned to the toString property. Next time the toString will be invoked, JS will start scanning the prototype chain all over again, and use the first occurance of the method (in the prototype). Let's clarify:
function SomeConstructor()
{
}
SomeConstructor.prototype.methd = function()
{
return true;
}
var f = new SomeConstructor();
var g = new SomeConstructor();
f.methd = function(){return false;};
g.methd();//returns true, still <-- method is gotten from the prototype
f.methd();//returns false <-- method is defined # instance level
delete (f.methd);
f.methd();//returns true, f doesn't have the method, but the prototype still does, so JS uses that.
Or even better, you can even replace an instance's method by a method from another prototype:
f.methd = Object.prototype.valueOf;//for as long as you need
the last example is pointless, because f has the valueOf method already: its inheritance chain looks like this: var f ---> SomeConstructor ---> Object, giving you access to all Object.prototype methods, too! Neat, isn't it?
These are just dummy examples, but I hope you see this is one of those features that make JS an incredibly flexible (sometimes too flexible, I must admit) and expressive language.

In first case the function will be created for each instance and set to the foo property in the object. In second case it is shared function. When you call obj.prop then it looks for it in object itself, if it is not there, then it looks for it in proto object and so on, it is called chain of prototypes.
For example this code provides foo:
function MyClass() {
this.foo = function () {};
}
var myVariable = new MyClass();
for (var i in myVariable) if (myVariable.hasOwnProperty(i)) console.log(i);
But this not:
function MyClass() {
}
MyClass.prototype.foo = function () {};
var myVariable = new MyClass();
for (var i in myVariable) if (myVariable.hasOwnProperty(i)) console.log(i);

Why use object.prototype.constructor in OOP javascript?

I've recently started reading up on OOP javascript and one thing that authors seem to skip over is when an object A has been declared and suddenly I see "A.prototype.constructor =A;
For example,
var A = function(){}; // This is the constructor of "A"
A.prototype.constructor = A;
A.prototype.value = 1;
A.prototype.test = function() { alert(this.value); }
var a = new A(); // create an instance of A
alert(a.value); // => 1
So I run the command in firebug "var A = function(){};"
and then "A.Constructor" Which reveals it's a function. I understand this.
I run the code "A.prototype.constructor = A;" and I thought this changes the A constructor from Function to A.
The constructor property of A has been changed right? Instead when I run "A.constructor" it gives me function () still.
What's the point?
I also see A.constructor.prototype.constructor.prototype.. what is going on?

If A inherit B using A.prototype = new B();, you need to reset the constructor property for the class A using A.prototype.constructor=A;, otherwise instances of A would have a constructor of B.
In your case, A.prototype.constructor === A will return true, so A.prototype.constructor = A did nothing.

You can quickly test out that that additional assignment does absolutely nothing:
var A = function() {};
A.prototype.constructor === A; // true -- why assign then?
Resetting the constructor property only makes sense if you've assigned a new prototype object to the class, overwriting the original constructor:
var A = function() {};
A.prototype = protoObject; // some object with members that you'd like to inherit
A.prototype.constructor = A; // reset constructor
In your case, the author might be blindly doing this as good practice, even in cases where it's not necessary.

This code if often use in JS classic inheritance pattern (the code is from JavaScript Patterns by Stoyan Stefanov):
function inherit(C, P) {
var F = function () {};
F.prototype = P.prototype;
C.prototype = new F();
C.uber = P.prototype;
C.prototype.constructor = C;
}
to assign right constructor to the child class.
In your case it did nothing, since A.prototype.constructor === A before assignment.

You might want to see my answer to similar question:
https://stackoverflow.com/a/19616652/207661
TL;DR: constructor is not an own property of an instance. So to make things look consistent JavaScript interpreter needs to set prototype.constructor to function itself. This feature can be used in functions that operates generically on many different types of objects.

According to MDN, All objects inherit a constructor property from their prototype:
Example 1:
var o = {};
o.constructor === Object; // true
..
Example2:
function Tree() {
}
var theTree = new Tree();
console.log(theTree.constructor === Tree ); // true
At runtime, it does not make any difference based on the value of the constructor property.
However, as the constructor property returns a reference to the Object function that created the instance's prototype, one should reset the constructor property when they assign a new prototype to the Object function.
var Forest = function() {};
Forest.prototype = theTree;
console.log(new Forest().constructor === Tree ); // true
Forest.prototype.constructor = Forest;
console.log(new Forest().constructor === Forest ); // true
https://jsfiddle.net/j1ub9sap/
For details: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/constructor

build an object without using of "new"

How can this object be rewritten so you don't need to declare it with "new"?
var Lang = new function(){
this.get = function(str, trans){
if(TRANSLATE[str]){
var str = TRANSLATE[str][LANG];
if(count_obj(trans) > 0){
for(var key in trans){
str = str.replace('%'+key+'%', trans[key]);
}
}
}
return str;
};
};
To something like this:
var Lang = {
get : function(){}
};

You wrote the solution to your own question in the question...this is a perfectly valid way to create an object in javascript:
var Lang = {
get: function(str, trans){
if(TRANSLATE[str]){
var str = TRANSLATE[str][LANG];
if(count_obj(trans) > 0){
for(var key in trans){
str = str.replace('%'+key+'%', trans[key]);
}
}
}
return str;
};
};
If you want private variables, the usual way to do that is to create a function with local variables that return the object with those variables encapsulated in a closure like so:
var Lang = (function() {
var p1; // private variable
var p2; // another private variable
return {
getP1: function () {
return p1;
},
setP1: function(value) {
p1 = value;
}
};
})();
Note that the function that creates the object you want is executed right away and returns the object with two private variables captured in the closure. Those variables will not be accessible from outside the object.

Patterns for Enforcing new
As mentioned already, constructors are still just functions but invoked with new.
What
happens if you forget new when you invoke a constructor? This is not going to cause
syntax or runtime errors but might lead to logical errors and unexpected behavior.
That’s because when you forget new, this inside the constructor will point to the global
object. (In browsers this will point to window.)
When your constructor has something like this.member and you invoke the constructor
without new, you’re actually creating a new property of the global object called
member and accessible through window.member or simply member. This behavior is highly
undesirable, because you know you should always strive for keeping the global namespace
clean.
// constructor
function Waffle() {
this.tastes = "yummy";
}
// a new object
var good_morning = new Waffle();
console.log(typeof good_morning); // "object"
console.log(good_morning.tastes); // "yummy"
// antipattern:
// forgotten `new`
var good_morning = Waffle();
console.log(typeof good_morning); // "undefined"
console.log(window.tastes); // "yummy"
Self-Invoking Constructor
To address the drawback of the previous pattern and have prototype properties available
to the instance objects, consider the following approach. In the constructor you
check whether this is an instance of your constructor, and if not, the constructor invokes
itself again, this time properly with new:
function Waffle() {
if (!(this instanceof Waffle)) {
return new Waffle();
}
this.tastes = "yummy";
}
Waffle.prototype.wantAnother = true;
// testing invocations
var first = new Waffle(),
second = Waffle();
console.log(first.tastes); // "yummy"
console.log(second.tastes); // "yummy"
console.log(first.wantAnother); // true
console.log(second.wantAnother); // true
Another general-purpose way to check the instance is to compare with
arguments.callee instead of hard-coding the constructor name.
if (!(this instanceof arguments.callee)) {
return new arguments.callee();
}
This pattern uses the fact that inside every function, an object called arguments is created
containing all parameters passed to the function when it was invoked. And arguments
has a property called callee, which points back to the function that was called. Be
aware that arguments.callee is not allowed in ES5’s strict mode, so it’s best if you limit
its future use and also remove any instances should you find them in existing code.
“JavaScript Patterns, by Stoyan Stefanov (O’Reilly). Copyright 2010 Yahoo!, Inc., 9780596806750.”

just leave the "new" out, rest is the same. :)
var Lang = function(){
...
}
Edit: example copy-paste from firebug, "dsa" is just an object, "dsaFunc" is a function, but you can do the same with them:
>>> var dsa = {};
undefined
>>> dsa.get = function(a){ return a+1;}
function()
>>> dsa.get(2)
3
>>> var dsaFunc = function(){};
undefined
>>> dsaFunc.get = function(a){ return a+1;}
function()
>>> dsaFunc.get(2)
3