Prototypical Inheritance - Why doesn't this work? - javascript

example = {};
example.Math = {
sin: function() {console.log('sin');},
cos: function() {console.log('cos');}
};
foo = function(){
sin();
};
foo.prototype = window.example.Math;
console.log(foo.prototype)
console.log(foo.cos)
console.log(foo())
http://jsfiddle.net/Mubqy/9/
From what I thought I understood references are searched for through the scope chain. So theoretically if I assign an object with functions defined to a prototype I should get all of those functions and fields of the object or so I thought. Where is the error in my understanding? Neither of the above scenarios work. Additionally the internal proto variable is not being updated. Obviously that's why it is not working, but why doesn't my assignment to the prototype work?

I think you need to realize that the prototyped method needs to be run with the new keyword, and it needs to use this to access its variables and methods, like this:
example = {};
example.Math = {
sin: function() {console.log('sin');},
cos: function() {console.log('cos');}
};
foo = function(){
this.sin();
};
foo.prototype = window.example.Math;
console.log(foo.prototype)
console.log(foo.cos)
console.log(new foo())

You missed two very crucial aspects of prototype.
A prototype acts as the parent of the object
You must initiate an instance of your object
First, use the keyword this to access object's methods and variables:
var foo = function(){
this.sin();
};
Than initiate an instance of the object using the keyword new:
new foo();
Just like OOP, only not :)
http://jsfiddle.net/46fPe/

Related

Property of object not returning the expected value 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

Why create JavaScript object methods outside of the object declaration instead of inside? [duplicate]

What's the difference between
var A = function () {
this.x = function () {
//do something
};
};
and
var A = function () { };
A.prototype.x = function () {
//do something
};
The examples have very different outcomes.
Before looking at the differences, the following should be noted:
A constructor's prototype provides a way to share methods and values among instances via the instance's private [[Prototype]] property.
A function's this is set by how the function is called or by the use of bind (not discussed here). Where a function is called on an object (e.g. myObj.method()) then this within the method references the object. Where this is not set by the call or by the use of bind, it defaults to the global object (window in a browser) or in strict mode, remains undefined.
JavaScript is an object-oriented language, i.e. most values are objects, including functions. (Strings, numbers, and booleans are not objects.)
So here are the snippets in question:
var A = function () {
this.x = function () {
//do something
};
};
In this case, variable A is assigned a value that is a reference to a function. When that function is called using A(), the function's this isn't set by the call so it defaults to the global object and the expression this.x is effective window.x. The result is that a reference to the function expression on the right-hand side is assigned to window.x.
In the case of:
var A = function () { };
A.prototype.x = function () {
//do something
};
something very different occurs. In the first line, variable A is assigned a reference to a function. In JavaScript, all functions objects have a prototype property by default so there is no separate code to create an A.prototype object.
In the second line, A.prototype.x is assigned a reference to a function. This will create an x property if it doesn't exist, or assign a new value if it does. So the difference with the first example in which object's x property is involved in the expression.
Another example is below. It's similar to the first one (and maybe what you meant to ask about):
var A = new function () {
this.x = function () {
//do something
};
};
In this example, the new operator has been added before the function expression so that the function is called as a constructor. When called with new, the function's this is set to reference a new Object whose private [[Prototype]] property is set to reference the constructor's public prototype. So in the assignment statement, the x property will be created on this new object. When called as a constructor, a function returns its this object by default, so there is no need for a separate return this; statement.
To check that A has an x property:
console.log(A.x) // function () {
// //do something
// };
This is an uncommon use of new since the only way to reference the constructor is via A.constructor. It would be much more common to do:
var A = function () {
this.x = function () {
//do something
};
};
var a = new A();
Another way of achieving a similar result is to use an immediately invoked function expression:
var A = (function () {
this.x = function () {
//do something
};
}());
In this case, A assigned the return value of calling the function on the right-hand side. Here again, since this is not set in the call, it will reference the global object and this.x is effective window.x. Since the function doesn't return anything, A will have a value of undefined.
These differences between the two approaches also manifest if you're serializing and de-serializing your Javascript objects to/from JSON. Methods defined on an object's prototype are not serialized when you serialize the object, which can be convenient when for example you want to serialize just the data portions of an object, but not it's methods:
var A = function () {
this.objectsOwnProperties = "are serialized";
};
A.prototype.prototypeProperties = "are NOT serialized";
var instance = new A();
console.log(instance.prototypeProperties); // "are NOT serialized"
console.log(JSON.stringify(instance));
// {"objectsOwnProperties":"are serialized"}
Related questions:
What does it mean that JavaScript is a prototypal language?
What is the scope of a function in JavaScript?
How does the "this" keyword work?
Sidenote: There may not be any significant memory savings between the two approaches, however using the prototype to share methods and properties will likely use less memory than each instance having its own copy.
JavaScript isn't a low-level language. It may not be very valuable to think of prototyping or other inheritance patterns as a way to explicitly change the way memory is allocated.
As others have said the first version, using "this" results in every instance of the class A having its own independent copy of function method "x". Whereas using "prototype" will mean that each instance of class A will use the same copy of method "x".
Here is some code to show this subtle difference:
// x is a method assigned to the object using "this"
var A = function () {
this.x = function () { alert('A'); };
};
A.prototype.updateX = function( value ) {
this.x = function() { alert( value ); }
};
var a1 = new A();
var a2 = new A();
a1.x(); // Displays 'A'
a2.x(); // Also displays 'A'
a1.updateX('Z');
a1.x(); // Displays 'Z'
a2.x(); // Still displays 'A'
// Here x is a method assigned to the object using "prototype"
var B = function () { };
B.prototype.x = function () { alert('B'); };
B.prototype.updateX = function( value ) {
B.prototype.x = function() { alert( value ); }
}
var b1 = new B();
var b2 = new B();
b1.x(); // Displays 'B'
b2.x(); // Also displays 'B'
b1.updateX('Y');
b1.x(); // Displays 'Y'
b2.x(); // Also displays 'Y' because by using prototype we have changed it for all instances
As others have mentioned, there are various reasons to choose one method or the other. My sample is just meant to clearly demonstrate the difference.
Take these 2 examples:
var A = function() { this.hey = function() { alert('from A') } };
vs.
var A = function() {}
A.prototype.hey = function() { alert('from prototype') };
Most people here (especially the top-rated answers) tried to explain how they are different without explaining WHY. I think this is wrong and if you understand the fundamentals first, the difference will become obvious. Let's try to explain the fundamentals first...
a) A function is an object in JavaScript. EVERY object in JavaScript gets an internal property (meaning, you can't access it like other properties, except maybe in browsers like Chrome), often referred to as __proto__ (you can actually type anyObject.__proto__ in Chrome to see what it references. This is just that, a property, nothing more. A property in JavaScript = a variable inside an object, nothing more. What do variables do? They point to things.
So what does this __proto__ property points to? Well, usually another object (we'll explain why later). The only way to force JavaScript for the __proto__ property to NOT point to another object is to use var newObj = Object.create(null). Even if you do this, the __proto__ property STILL exists as a property of the object, just it doesn't point to another object, it points to null.
Here's where most people get confused:
When you create a new function in JavaScript (which is an object as well, remember?), the moment it is defined, JavaScript automatically creates a new property on that function called prototype. Try it:
var A = [];
A.prototype // undefined
A = function() {}
A.prototype // {} // got created when function() {} was defined
A.prototype is TOTALLY DIFFERENT from the __proto__ property. In our example, 'A' now has TWO properties called 'prototype' and __proto__ . This is a big confusion for people. prototype and __proto__ properties are in no way related, they're separate things pointing to separate values.
You may wonder: Why does JavaScript has __proto__ property created on every single object? Well, one word: delegation. When you call a property on an object and the object doesn't have it, then JavaScript looks for the object referenced by __proto__ to see if it maybe has it. If it doesn't have it, then it looks at that object's __proto__ property and so on...until the chain ends. Thus the name prototype chain. Of course, if __proto__ doesn't point to an object and instead points to null, well tough luck, JavaScript realizes that and will return you undefined for the property.
You may also wonder, why does JavaScript creates a property called prototype for a function when you define the function? Because it tries to fool you, yes fool you that it works like class-based languages.
Let's go on with our example and create an "object" out of A:
var a1 = new A();
There's something happening in the background when this thing happened. a1 is an ordinary variable which was assigned a new, empty object.
The fact that you used the operator new before a function invocation A() did something ADDITIONAL in the background. The new keyword created a new object which now references a1 and that object is empty. Here's what happening additionally:
We said that on each function definition there's a new property created called prototype (which you can access it, unlike with the __proto__ property) created? Well, that property is being used now.
So we're now at the point where we have a freshly baked empty a1 object. We said that all objects in JavaScript have an internal __proto__ property which points to something (a1 also has it), whether it's null or another object. What the new operator does is that it sets that __proto__ property to point to the function's prototype property. Read that again. It's basically this:
a1.__proto__ = A.prototype;
We said that A.prototype is nothing more than an empty object (unless we change it to something else before defining a1). So now basically a1.__proto__ points to the same thing A.prototype points to, which is that empty object. They both point to the same object which was created when this line happened:
A = function() {} // JS: cool. let's also create A.prototype pointing to empty {}
Now, there's another thing happening when var a1 = new A() statement is processed. Basically A() is executed and if A is something like this:
var A = function() { this.hey = function() { alert('from A') } };
All that stuff inside function() { } is going to execute. When you reach the this.hey.. line, this is changed to a1 and you get this:
a1.hey = function() { alert('from A') }
I won't cover why this changes to a1 but this is a great answer to learn more.
So to summarize, when you do var a1 = new A() there are 3 things happening in the background:
A totally new empty object is created and assigned to a1. a1 = {}
a1.__proto__ property is assigned to point at the same thing as A.prototype points to (another empty object {} )
The function A() is being executed with this set to the new, empty object created in step 1 (read the answer I referenced above as to why this changes to a1)
Now, let's try to create another object:
var a2 = new A();
Steps 1,2,3 will repeat. Do you notice something? The key word is repeat. Step 1: a2 will be a new empty object, step 2: its __proto__ property will point to the same thing A.prototype points to and most importantly, step 3: function A() is AGAIN executed, which means that a2 will get hey property containing a function. a1 and a2 have two SEPARATE properties named hey which point to 2 SEPARATE functions! We now have duplicate functions in same two different objects doing the same thing, oops...You can imagine the memory implications of this if we have 1000 objects created with new A, after all functions declarations take more memory than something like the number 2. So how do we prevent this?
Remember why the __proto__ property exists on every object? So that if you retrieve the yoMan property on a1 (which doesn't exist), its __proto__ property will be consulted, which if it's an object (and is most cases it is), it will check if it contains yoMan, and if it doesn't, it will consult that object's __proto__ etc. If it does, it will take that property value and display it to you.
So someone decided to use this fact + the fact that when you create a1, its __proto__ property points to the same (empty) object A.prototype points to and do this:
var A = function() {}
A.prototype.hey = function() { alert('from prototype') };
Cool! Now, when you create a1, it again goes through all of the 3 steps above, and in step 3, it doesn't do anything, since function A() has nothing to execute. And if we do:
a1.hey
It will see that a1 does not contain hey and it will check its __proto__ property object to see if it has it, which is the case.
With this approach we eliminate the part from step 3 where functions are duplicated on each new object creation. Instead of a1 and a2 having a separate hey property, now NONE of them has it. Which, I guess, you figured out yourself by now. That's the nice thing...if you understand __proto__ and Function.prototype, questions like these will be pretty obvious.
NOTE: Some people tend to not call the internal Prototype property as __proto__, I've used this name through the post to distinguish it clearly to the Functional.prototype property as two different things.
In most cases they are essentially the same, but the second version saves memory because there is only one instance of the function instead of a separate function for each object.
A reason to use the first form is to access "private members". For example:
var A = function () {
var private_var = ...;
this.x = function () {
return private_var;
};
this.setX = function (new_x) {
private_var = new_x;
};
};
Because of javascript's scoping rules, private_var is available to the function assigned to this.x, but not outside the object.
The first example changes the interface for that object only. The second example changes the interface for all object of that class.
The ultimate problem with using this instead of prototype is that when overriding a method, the constructor of the base class will still refer to the overridden method. Consider this:
BaseClass = function() {
var text = null;
this.setText = function(value) {
text = value + " BaseClass!";
};
this.getText = function() {
return text;
};
this.setText("Hello"); // This always calls BaseClass.setText()
};
SubClass = function() {
// setText is not overridden yet,
// so the constructor calls the superclass' method
BaseClass.call(this);
// Keeping a reference to the superclass' method
var super_setText = this.setText;
// Overriding
this.setText = function(value) {
super_setText.call(this, "SubClass says: " + value);
};
};
SubClass.prototype = new BaseClass();
var subClass = new SubClass();
console.log(subClass.getText()); // Hello BaseClass!
subClass.setText("Hello"); // setText is already overridden
console.log(subClass.getText()); // SubClass says: Hello BaseClass!
versus:
BaseClass = function() {
this.setText("Hello"); // This calls the overridden method
};
BaseClass.prototype.setText = function(value) {
this.text = value + " BaseClass!";
};
BaseClass.prototype.getText = function() {
return this.text;
};
SubClass = function() {
// setText is already overridden, so this works as expected
BaseClass.call(this);
};
SubClass.prototype = new BaseClass();
SubClass.prototype.setText = function(value) {
BaseClass.prototype.setText.call(this, "SubClass says: " + value);
};
var subClass = new SubClass();
console.log(subClass.getText()); // SubClass says: Hello BaseClass!
If you think this is not a problem, then it depends on whether you can live without private variables, and whether you are experienced enough to know a leak when you see one. Also, having to put the constructor logic after the method definitions is inconvenient.
var A = function (param1) {
var privateVar = null; // Private variable
// Calling this.setPrivateVar(param1) here would be an error
this.setPrivateVar = function (value) {
privateVar = value;
console.log("setPrivateVar value set to: " + value);
// param1 is still here, possible memory leak
console.log("setPrivateVar has param1: " + param1);
};
// The constructor logic starts here possibly after
// many lines of code that define methods
this.setPrivateVar(param1); // This is valid
};
var a = new A(0);
// setPrivateVar value set to: 0
// setPrivateVar has param1: 0
a.setPrivateVar(1);
//setPrivateVar value set to: 1
//setPrivateVar has param1: 0
versus:
var A = function (param1) {
this.setPublicVar(param1); // This is valid
};
A.prototype.setPublicVar = function (value) {
this.publicVar = value; // No private variable
};
var a = new A(0);
a.setPublicVar(1);
console.log(a.publicVar); // 1
Every object is linked to a prototype object. When trying to access a property that does not exist, JavaScript will look in the object's prototype object for that property and return it if it exists.
The prototype property of a function constructor refers to the prototype object of all instances created with that function when using new.
In your first example, you are adding a property x to each instance created with the A function.
var A = function () {
this.x = function () {
//do something
};
};
var a = new A(); // constructor function gets executed
// newly created object gets an 'x' property
// which is a function
a.x(); // and can be called like this
In the second example you are adding a property to the prototype object that all the instances created with A point to.
var A = function () { };
A.prototype.x = function () {
//do something
};
var a = new A(); // constructor function gets executed
// which does nothing in this example
a.x(); // you are trying to access the 'x' property of an instance of 'A'
// which does not exist
// so JavaScript looks for that property in the prototype object
// that was defined using the 'prototype' property of the constructor
In conclusion, in the first example a copy of the function is assigned to each instance. In the second example a single copy of the function is shared by all instances.
What's the difference? => A lot.
I think, the this version is used to enable encapsulation, i.e. data hiding.
It helps to manipulate private variables.
Let us look at the following example:
var AdultPerson = function() {
var age;
this.setAge = function(val) {
// some housekeeping
age = val >= 18 && val;
};
this.getAge = function() {
return age;
};
this.isValid = function() {
return !!age;
};
};
Now, the prototype structure can be applied as following:
Different adults have different ages, but all of the adults get the same rights.
So, we add it using prototype, rather than this.
AdultPerson.prototype.getRights = function() {
// Should be valid
return this.isValid() && ['Booze', 'Drive'];
};
Lets look at the implementation now.
var p1 = new AdultPerson;
p1.setAge(12); // ( age = false )
console.log(p1.getRights()); // false ( Kid alert! )
p1.setAge(19); // ( age = 19 )
console.log(p1.getRights()); // ['Booze', 'Drive'] ( Welcome AdultPerson )
var p2 = new AdultPerson;
p2.setAge(45);
console.log(p2.getRights()); // The same getRights() method, *** not a new copy of it ***
Hope this helps.
I know this has been answered to death but I'd like to show an actual example of speed differences.
Function directly on object:
function ExampleFn() {
this.print = function() {
console.log("Calling print! ");
}
}
var objects = [];
console.time('x');
for (let i = 0; i < 2000000; i++) {
objects.push(new ExampleFn());
}
console.timeEnd('x');
//x: 1151.960693359375ms
Function on prototype:
function ExampleFn() {
}
ExampleFn.prototype.print = function() {
console.log("Calling print!");
}
var objects = [];
console.time('y');
for (let i = 0; i < 2000000; i++) {
objects.push(new ExampleFn());
}
console.timeEnd('y');
//x: 617.866943359375ms
Here we're creating 2,000,000 new objects with a print method in Chrome. We're storing every object in an array. Putting print on the prototype takes about 1/2 as long.
Prototype is the template of the class; which applies to all future instances of it. Whereas this is the particular instance of the object.
Let me give you a more comprehensive answer that I learned during a JavaScript training course.
Most answers mentioned the difference already, i.e. when prototyping the function is shared with all (future) instances. Whereas declaring the function in the class will create a copy for each instance.
In general there is no right or wrong, it's more a matter of taste or a design decision depending on your requirements. The prototype however is the technique that is used to develop in an object oriented manner, as I hope you'll see at the end of this answer.
You showed two patterns in your question. I will try to explain two more and try to explain the differences if relevant. Feel free to edit/extend.
In all examples it is about a car object that has a location and can move.
Object Decorator pattern
Not sure if this pattern is still relevant nowadays, but it exists. And it is good to know about it.
You simply pass an object and a property to the decorator function. The decorator returns the object with property and method.
var carlike = function(obj, loc) {
obj.loc = loc;
obj.move = function() {
obj.loc++;
};
return obj;
};
var amy = carlike({}, 1);
amy.move();
var ben = carlike({}, 9);
ben.move();
Functional Classes
A function in JavaScript is a specialised object. In addition to being invoked, a function can store properties like any other object.
In this case Car is a function (also think object) that can be invoked as you are used to do. It has a property methods (which is an object with a move function). When Car is invoked the extend function is called, which does some magic, and extends the Car function (think object) with the methods defined within methods.
This example, though different, comes closest to the first example in the question.
var Car = function(loc) {
var obj = {loc: loc};
extend(obj, Car.methods);
return obj;
};
Car.methods = {
move : function() {
this.loc++;
}
};
var amy = Car(1);
amy.move();
var ben = Car(9);
ben.move();
Prototypal Classes
The first two patterns allow a discussion of using techniques to define shared methods or using methods that are defined inline in the body of the constructor. In both cases every instance has its own move function.
The prototypal pattern does not lend itself well to the same examination, because function sharing via a prototype delegation is the very goal for the prototypal pattern. As others pointed out, it is expected to have a better memory footprint.
However there is one point interesting to know:
Every prototype object has has a convenience property constructor, which points back to the function (think object) it came attached to.
Concerning the last three lines:
In this example Car links to the prototype object, which links via constructor to Car itself, i.e. Car.prototype.constructor is Car itself. This allows you to figure out which constructor function built a certain object.
amy.constructor's lookup fails and thus is delegated to Car.prototype, which does have the constructor property. And so amy.constructor is Car.
Furthermore, amy is an instanceof Car. The instanceof operator works by seeing if the right operand's prototype object (Car) can be found anywhere in the left operand's prototype (amy) chain.
var Car = function(loc) {
var obj = Object.create(Car.prototype);
obj.loc = loc;
return obj;
};
Car.prototype.move = function() {
this.loc++;
};
var amy = Car(1);
amy.move();
var ben = Car(9);
ben.move();
console.log(Car.prototype.constructor);
console.log(amy.constructor);
console.log(amy instanceof Car);
Some developers can be confused in the beginning. See below example:
var Dog = function() {
return {legs: 4, bark: alert};
};
var fido = Dog();
console.log(fido instanceof Dog);
The instanceof operator returns false, because Dog's prototype cannot be found anywhere in fido's prototype chain. fido is a simple object that is created with an object literal, i.e. it just delegates to Object.prototype.
Pseudoclassical patterns
This is really just another form of the prototypal pattern in simplified form and more familiar to do those who program in Java for example, since it uses the new constructor.
It does the same as in the prototypal pattern really, it is just syntactic sugar overtop of the prototypal pattern.
However, the primary difference is that there are optimizations implemented in JavaScript engines that only apply when using the pseudoclassical pattern. Think of the pseudoclassical pattern a probably faster version of the prototypal pattern; the object relations in both examples are the same.
var Car = function(loc) {
this.loc = loc;
};
Car.prototype.move = function() {
this.loc++;
};
var amy = new Car(1);
amy.move();
var ben = new Car(9);
ben.move();
Finally, it should not be too difficult to realize how object oriented programming can be done. There are two sections.
One section that defines common properties/methods in the prototype (chain).
And another section where you put the definitions that distinguish the objects from each other (loc variable in the examples).
This is what allows us to apply concepts like superclass or subclass in JavaScript.
Feel free to add or edit. Once more complete I could make this a community wiki maybe.
I believe that #Matthew Crumley is right. They are functionally, if not structurally, equivalent. If you use Firebug to look at the objects that are created using new, you can see that they are the same. However, my preference would be the following. I'm guessing that it just seems more like what I'm used to in C#/Java. That is, define the class, define the fields, constructor, and methods.
var A = function() {};
A.prototype = {
_instance_var: 0,
initialize: function(v) { this._instance_var = v; },
x: function() { alert(this._instance_var); }
};
EDIT Didn't mean to imply that the scope of the variable was private, I was just trying to illustrate how I define my classes in javascript. Variable name has been changed to reflect this.
As discussed in other answers, it's really a performance consideration because the function in the prototype is shared with all of the instantiations - rather than the function being created for each instantiation.
I put together a jsperf to show this. There is a dramatic difference in the time it takes to instantiate the class, although it is really only relevant if you are making many instances.
http://jsperf.com/functions-in-constructor-vs-prototype
Think about statically typed language, things on prototype are static and things on this are instance related.
When you use prototype, the function will only be loaded only once into memory (independently on the amount of objects you create) and you can override the function whenever you want.

Is there any difference between ES6 class constructor functions and method? [duplicate]

What's the difference between
var A = function () {
this.x = function () {
//do something
};
};
and
var A = function () { };
A.prototype.x = function () {
//do something
};
The examples have very different outcomes.
Before looking at the differences, the following should be noted:
A constructor's prototype provides a way to share methods and values among instances via the instance's private [[Prototype]] property.
A function's this is set by how the function is called or by the use of bind (not discussed here). Where a function is called on an object (e.g. myObj.method()) then this within the method references the object. Where this is not set by the call or by the use of bind, it defaults to the global object (window in a browser) or in strict mode, remains undefined.
JavaScript is an object-oriented language, i.e. most values are objects, including functions. (Strings, numbers, and booleans are not objects.)
So here are the snippets in question:
var A = function () {
this.x = function () {
//do something
};
};
In this case, variable A is assigned a value that is a reference to a function. When that function is called using A(), the function's this isn't set by the call so it defaults to the global object and the expression this.x is effective window.x. The result is that a reference to the function expression on the right-hand side is assigned to window.x.
In the case of:
var A = function () { };
A.prototype.x = function () {
//do something
};
something very different occurs. In the first line, variable A is assigned a reference to a function. In JavaScript, all functions objects have a prototype property by default so there is no separate code to create an A.prototype object.
In the second line, A.prototype.x is assigned a reference to a function. This will create an x property if it doesn't exist, or assign a new value if it does. So the difference with the first example in which object's x property is involved in the expression.
Another example is below. It's similar to the first one (and maybe what you meant to ask about):
var A = new function () {
this.x = function () {
//do something
};
};
In this example, the new operator has been added before the function expression so that the function is called as a constructor. When called with new, the function's this is set to reference a new Object whose private [[Prototype]] property is set to reference the constructor's public prototype. So in the assignment statement, the x property will be created on this new object. When called as a constructor, a function returns its this object by default, so there is no need for a separate return this; statement.
To check that A has an x property:
console.log(A.x) // function () {
// //do something
// };
This is an uncommon use of new since the only way to reference the constructor is via A.constructor. It would be much more common to do:
var A = function () {
this.x = function () {
//do something
};
};
var a = new A();
Another way of achieving a similar result is to use an immediately invoked function expression:
var A = (function () {
this.x = function () {
//do something
};
}());
In this case, A assigned the return value of calling the function on the right-hand side. Here again, since this is not set in the call, it will reference the global object and this.x is effective window.x. Since the function doesn't return anything, A will have a value of undefined.
These differences between the two approaches also manifest if you're serializing and de-serializing your Javascript objects to/from JSON. Methods defined on an object's prototype are not serialized when you serialize the object, which can be convenient when for example you want to serialize just the data portions of an object, but not it's methods:
var A = function () {
this.objectsOwnProperties = "are serialized";
};
A.prototype.prototypeProperties = "are NOT serialized";
var instance = new A();
console.log(instance.prototypeProperties); // "are NOT serialized"
console.log(JSON.stringify(instance));
// {"objectsOwnProperties":"are serialized"}
Related questions:
What does it mean that JavaScript is a prototypal language?
What is the scope of a function in JavaScript?
How does the "this" keyword work?
Sidenote: There may not be any significant memory savings between the two approaches, however using the prototype to share methods and properties will likely use less memory than each instance having its own copy.
JavaScript isn't a low-level language. It may not be very valuable to think of prototyping or other inheritance patterns as a way to explicitly change the way memory is allocated.
As others have said the first version, using "this" results in every instance of the class A having its own independent copy of function method "x". Whereas using "prototype" will mean that each instance of class A will use the same copy of method "x".
Here is some code to show this subtle difference:
// x is a method assigned to the object using "this"
var A = function () {
this.x = function () { alert('A'); };
};
A.prototype.updateX = function( value ) {
this.x = function() { alert( value ); }
};
var a1 = new A();
var a2 = new A();
a1.x(); // Displays 'A'
a2.x(); // Also displays 'A'
a1.updateX('Z');
a1.x(); // Displays 'Z'
a2.x(); // Still displays 'A'
// Here x is a method assigned to the object using "prototype"
var B = function () { };
B.prototype.x = function () { alert('B'); };
B.prototype.updateX = function( value ) {
B.prototype.x = function() { alert( value ); }
}
var b1 = new B();
var b2 = new B();
b1.x(); // Displays 'B'
b2.x(); // Also displays 'B'
b1.updateX('Y');
b1.x(); // Displays 'Y'
b2.x(); // Also displays 'Y' because by using prototype we have changed it for all instances
As others have mentioned, there are various reasons to choose one method or the other. My sample is just meant to clearly demonstrate the difference.
Take these 2 examples:
var A = function() { this.hey = function() { alert('from A') } };
vs.
var A = function() {}
A.prototype.hey = function() { alert('from prototype') };
Most people here (especially the top-rated answers) tried to explain how they are different without explaining WHY. I think this is wrong and if you understand the fundamentals first, the difference will become obvious. Let's try to explain the fundamentals first...
a) A function is an object in JavaScript. EVERY object in JavaScript gets an internal property (meaning, you can't access it like other properties, except maybe in browsers like Chrome), often referred to as __proto__ (you can actually type anyObject.__proto__ in Chrome to see what it references. This is just that, a property, nothing more. A property in JavaScript = a variable inside an object, nothing more. What do variables do? They point to things.
So what does this __proto__ property points to? Well, usually another object (we'll explain why later). The only way to force JavaScript for the __proto__ property to NOT point to another object is to use var newObj = Object.create(null). Even if you do this, the __proto__ property STILL exists as a property of the object, just it doesn't point to another object, it points to null.
Here's where most people get confused:
When you create a new function in JavaScript (which is an object as well, remember?), the moment it is defined, JavaScript automatically creates a new property on that function called prototype. Try it:
var A = [];
A.prototype // undefined
A = function() {}
A.prototype // {} // got created when function() {} was defined
A.prototype is TOTALLY DIFFERENT from the __proto__ property. In our example, 'A' now has TWO properties called 'prototype' and __proto__ . This is a big confusion for people. prototype and __proto__ properties are in no way related, they're separate things pointing to separate values.
You may wonder: Why does JavaScript has __proto__ property created on every single object? Well, one word: delegation. When you call a property on an object and the object doesn't have it, then JavaScript looks for the object referenced by __proto__ to see if it maybe has it. If it doesn't have it, then it looks at that object's __proto__ property and so on...until the chain ends. Thus the name prototype chain. Of course, if __proto__ doesn't point to an object and instead points to null, well tough luck, JavaScript realizes that and will return you undefined for the property.
You may also wonder, why does JavaScript creates a property called prototype for a function when you define the function? Because it tries to fool you, yes fool you that it works like class-based languages.
Let's go on with our example and create an "object" out of A:
var a1 = new A();
There's something happening in the background when this thing happened. a1 is an ordinary variable which was assigned a new, empty object.
The fact that you used the operator new before a function invocation A() did something ADDITIONAL in the background. The new keyword created a new object which now references a1 and that object is empty. Here's what happening additionally:
We said that on each function definition there's a new property created called prototype (which you can access it, unlike with the __proto__ property) created? Well, that property is being used now.
So we're now at the point where we have a freshly baked empty a1 object. We said that all objects in JavaScript have an internal __proto__ property which points to something (a1 also has it), whether it's null or another object. What the new operator does is that it sets that __proto__ property to point to the function's prototype property. Read that again. It's basically this:
a1.__proto__ = A.prototype;
We said that A.prototype is nothing more than an empty object (unless we change it to something else before defining a1). So now basically a1.__proto__ points to the same thing A.prototype points to, which is that empty object. They both point to the same object which was created when this line happened:
A = function() {} // JS: cool. let's also create A.prototype pointing to empty {}
Now, there's another thing happening when var a1 = new A() statement is processed. Basically A() is executed and if A is something like this:
var A = function() { this.hey = function() { alert('from A') } };
All that stuff inside function() { } is going to execute. When you reach the this.hey.. line, this is changed to a1 and you get this:
a1.hey = function() { alert('from A') }
I won't cover why this changes to a1 but this is a great answer to learn more.
So to summarize, when you do var a1 = new A() there are 3 things happening in the background:
A totally new empty object is created and assigned to a1. a1 = {}
a1.__proto__ property is assigned to point at the same thing as A.prototype points to (another empty object {} )
The function A() is being executed with this set to the new, empty object created in step 1 (read the answer I referenced above as to why this changes to a1)
Now, let's try to create another object:
var a2 = new A();
Steps 1,2,3 will repeat. Do you notice something? The key word is repeat. Step 1: a2 will be a new empty object, step 2: its __proto__ property will point to the same thing A.prototype points to and most importantly, step 3: function A() is AGAIN executed, which means that a2 will get hey property containing a function. a1 and a2 have two SEPARATE properties named hey which point to 2 SEPARATE functions! We now have duplicate functions in same two different objects doing the same thing, oops...You can imagine the memory implications of this if we have 1000 objects created with new A, after all functions declarations take more memory than something like the number 2. So how do we prevent this?
Remember why the __proto__ property exists on every object? So that if you retrieve the yoMan property on a1 (which doesn't exist), its __proto__ property will be consulted, which if it's an object (and is most cases it is), it will check if it contains yoMan, and if it doesn't, it will consult that object's __proto__ etc. If it does, it will take that property value and display it to you.
So someone decided to use this fact + the fact that when you create a1, its __proto__ property points to the same (empty) object A.prototype points to and do this:
var A = function() {}
A.prototype.hey = function() { alert('from prototype') };
Cool! Now, when you create a1, it again goes through all of the 3 steps above, and in step 3, it doesn't do anything, since function A() has nothing to execute. And if we do:
a1.hey
It will see that a1 does not contain hey and it will check its __proto__ property object to see if it has it, which is the case.
With this approach we eliminate the part from step 3 where functions are duplicated on each new object creation. Instead of a1 and a2 having a separate hey property, now NONE of them has it. Which, I guess, you figured out yourself by now. That's the nice thing...if you understand __proto__ and Function.prototype, questions like these will be pretty obvious.
NOTE: Some people tend to not call the internal Prototype property as __proto__, I've used this name through the post to distinguish it clearly to the Functional.prototype property as two different things.
In most cases they are essentially the same, but the second version saves memory because there is only one instance of the function instead of a separate function for each object.
A reason to use the first form is to access "private members". For example:
var A = function () {
var private_var = ...;
this.x = function () {
return private_var;
};
this.setX = function (new_x) {
private_var = new_x;
};
};
Because of javascript's scoping rules, private_var is available to the function assigned to this.x, but not outside the object.
The first example changes the interface for that object only. The second example changes the interface for all object of that class.
The ultimate problem with using this instead of prototype is that when overriding a method, the constructor of the base class will still refer to the overridden method. Consider this:
BaseClass = function() {
var text = null;
this.setText = function(value) {
text = value + " BaseClass!";
};
this.getText = function() {
return text;
};
this.setText("Hello"); // This always calls BaseClass.setText()
};
SubClass = function() {
// setText is not overridden yet,
// so the constructor calls the superclass' method
BaseClass.call(this);
// Keeping a reference to the superclass' method
var super_setText = this.setText;
// Overriding
this.setText = function(value) {
super_setText.call(this, "SubClass says: " + value);
};
};
SubClass.prototype = new BaseClass();
var subClass = new SubClass();
console.log(subClass.getText()); // Hello BaseClass!
subClass.setText("Hello"); // setText is already overridden
console.log(subClass.getText()); // SubClass says: Hello BaseClass!
versus:
BaseClass = function() {
this.setText("Hello"); // This calls the overridden method
};
BaseClass.prototype.setText = function(value) {
this.text = value + " BaseClass!";
};
BaseClass.prototype.getText = function() {
return this.text;
};
SubClass = function() {
// setText is already overridden, so this works as expected
BaseClass.call(this);
};
SubClass.prototype = new BaseClass();
SubClass.prototype.setText = function(value) {
BaseClass.prototype.setText.call(this, "SubClass says: " + value);
};
var subClass = new SubClass();
console.log(subClass.getText()); // SubClass says: Hello BaseClass!
If you think this is not a problem, then it depends on whether you can live without private variables, and whether you are experienced enough to know a leak when you see one. Also, having to put the constructor logic after the method definitions is inconvenient.
var A = function (param1) {
var privateVar = null; // Private variable
// Calling this.setPrivateVar(param1) here would be an error
this.setPrivateVar = function (value) {
privateVar = value;
console.log("setPrivateVar value set to: " + value);
// param1 is still here, possible memory leak
console.log("setPrivateVar has param1: " + param1);
};
// The constructor logic starts here possibly after
// many lines of code that define methods
this.setPrivateVar(param1); // This is valid
};
var a = new A(0);
// setPrivateVar value set to: 0
// setPrivateVar has param1: 0
a.setPrivateVar(1);
//setPrivateVar value set to: 1
//setPrivateVar has param1: 0
versus:
var A = function (param1) {
this.setPublicVar(param1); // This is valid
};
A.prototype.setPublicVar = function (value) {
this.publicVar = value; // No private variable
};
var a = new A(0);
a.setPublicVar(1);
console.log(a.publicVar); // 1
Every object is linked to a prototype object. When trying to access a property that does not exist, JavaScript will look in the object's prototype object for that property and return it if it exists.
The prototype property of a function constructor refers to the prototype object of all instances created with that function when using new.
In your first example, you are adding a property x to each instance created with the A function.
var A = function () {
this.x = function () {
//do something
};
};
var a = new A(); // constructor function gets executed
// newly created object gets an 'x' property
// which is a function
a.x(); // and can be called like this
In the second example you are adding a property to the prototype object that all the instances created with A point to.
var A = function () { };
A.prototype.x = function () {
//do something
};
var a = new A(); // constructor function gets executed
// which does nothing in this example
a.x(); // you are trying to access the 'x' property of an instance of 'A'
// which does not exist
// so JavaScript looks for that property in the prototype object
// that was defined using the 'prototype' property of the constructor
In conclusion, in the first example a copy of the function is assigned to each instance. In the second example a single copy of the function is shared by all instances.
What's the difference? => A lot.
I think, the this version is used to enable encapsulation, i.e. data hiding.
It helps to manipulate private variables.
Let us look at the following example:
var AdultPerson = function() {
var age;
this.setAge = function(val) {
// some housekeeping
age = val >= 18 && val;
};
this.getAge = function() {
return age;
};
this.isValid = function() {
return !!age;
};
};
Now, the prototype structure can be applied as following:
Different adults have different ages, but all of the adults get the same rights.
So, we add it using prototype, rather than this.
AdultPerson.prototype.getRights = function() {
// Should be valid
return this.isValid() && ['Booze', 'Drive'];
};
Lets look at the implementation now.
var p1 = new AdultPerson;
p1.setAge(12); // ( age = false )
console.log(p1.getRights()); // false ( Kid alert! )
p1.setAge(19); // ( age = 19 )
console.log(p1.getRights()); // ['Booze', 'Drive'] ( Welcome AdultPerson )
var p2 = new AdultPerson;
p2.setAge(45);
console.log(p2.getRights()); // The same getRights() method, *** not a new copy of it ***
Hope this helps.
I know this has been answered to death but I'd like to show an actual example of speed differences.
Function directly on object:
function ExampleFn() {
this.print = function() {
console.log("Calling print! ");
}
}
var objects = [];
console.time('x');
for (let i = 0; i < 2000000; i++) {
objects.push(new ExampleFn());
}
console.timeEnd('x');
//x: 1151.960693359375ms
Function on prototype:
function ExampleFn() {
}
ExampleFn.prototype.print = function() {
console.log("Calling print!");
}
var objects = [];
console.time('y');
for (let i = 0; i < 2000000; i++) {
objects.push(new ExampleFn());
}
console.timeEnd('y');
//x: 617.866943359375ms
Here we're creating 2,000,000 new objects with a print method in Chrome. We're storing every object in an array. Putting print on the prototype takes about 1/2 as long.
Prototype is the template of the class; which applies to all future instances of it. Whereas this is the particular instance of the object.
Let me give you a more comprehensive answer that I learned during a JavaScript training course.
Most answers mentioned the difference already, i.e. when prototyping the function is shared with all (future) instances. Whereas declaring the function in the class will create a copy for each instance.
In general there is no right or wrong, it's more a matter of taste or a design decision depending on your requirements. The prototype however is the technique that is used to develop in an object oriented manner, as I hope you'll see at the end of this answer.
You showed two patterns in your question. I will try to explain two more and try to explain the differences if relevant. Feel free to edit/extend.
In all examples it is about a car object that has a location and can move.
Object Decorator pattern
Not sure if this pattern is still relevant nowadays, but it exists. And it is good to know about it.
You simply pass an object and a property to the decorator function. The decorator returns the object with property and method.
var carlike = function(obj, loc) {
obj.loc = loc;
obj.move = function() {
obj.loc++;
};
return obj;
};
var amy = carlike({}, 1);
amy.move();
var ben = carlike({}, 9);
ben.move();
Functional Classes
A function in JavaScript is a specialised object. In addition to being invoked, a function can store properties like any other object.
In this case Car is a function (also think object) that can be invoked as you are used to do. It has a property methods (which is an object with a move function). When Car is invoked the extend function is called, which does some magic, and extends the Car function (think object) with the methods defined within methods.
This example, though different, comes closest to the first example in the question.
var Car = function(loc) {
var obj = {loc: loc};
extend(obj, Car.methods);
return obj;
};
Car.methods = {
move : function() {
this.loc++;
}
};
var amy = Car(1);
amy.move();
var ben = Car(9);
ben.move();
Prototypal Classes
The first two patterns allow a discussion of using techniques to define shared methods or using methods that are defined inline in the body of the constructor. In both cases every instance has its own move function.
The prototypal pattern does not lend itself well to the same examination, because function sharing via a prototype delegation is the very goal for the prototypal pattern. As others pointed out, it is expected to have a better memory footprint.
However there is one point interesting to know:
Every prototype object has has a convenience property constructor, which points back to the function (think object) it came attached to.
Concerning the last three lines:
In this example Car links to the prototype object, which links via constructor to Car itself, i.e. Car.prototype.constructor is Car itself. This allows you to figure out which constructor function built a certain object.
amy.constructor's lookup fails and thus is delegated to Car.prototype, which does have the constructor property. And so amy.constructor is Car.
Furthermore, amy is an instanceof Car. The instanceof operator works by seeing if the right operand's prototype object (Car) can be found anywhere in the left operand's prototype (amy) chain.
var Car = function(loc) {
var obj = Object.create(Car.prototype);
obj.loc = loc;
return obj;
};
Car.prototype.move = function() {
this.loc++;
};
var amy = Car(1);
amy.move();
var ben = Car(9);
ben.move();
console.log(Car.prototype.constructor);
console.log(amy.constructor);
console.log(amy instanceof Car);
Some developers can be confused in the beginning. See below example:
var Dog = function() {
return {legs: 4, bark: alert};
};
var fido = Dog();
console.log(fido instanceof Dog);
The instanceof operator returns false, because Dog's prototype cannot be found anywhere in fido's prototype chain. fido is a simple object that is created with an object literal, i.e. it just delegates to Object.prototype.
Pseudoclassical patterns
This is really just another form of the prototypal pattern in simplified form and more familiar to do those who program in Java for example, since it uses the new constructor.
It does the same as in the prototypal pattern really, it is just syntactic sugar overtop of the prototypal pattern.
However, the primary difference is that there are optimizations implemented in JavaScript engines that only apply when using the pseudoclassical pattern. Think of the pseudoclassical pattern a probably faster version of the prototypal pattern; the object relations in both examples are the same.
var Car = function(loc) {
this.loc = loc;
};
Car.prototype.move = function() {
this.loc++;
};
var amy = new Car(1);
amy.move();
var ben = new Car(9);
ben.move();
Finally, it should not be too difficult to realize how object oriented programming can be done. There are two sections.
One section that defines common properties/methods in the prototype (chain).
And another section where you put the definitions that distinguish the objects from each other (loc variable in the examples).
This is what allows us to apply concepts like superclass or subclass in JavaScript.
Feel free to add or edit. Once more complete I could make this a community wiki maybe.
I believe that #Matthew Crumley is right. They are functionally, if not structurally, equivalent. If you use Firebug to look at the objects that are created using new, you can see that they are the same. However, my preference would be the following. I'm guessing that it just seems more like what I'm used to in C#/Java. That is, define the class, define the fields, constructor, and methods.
var A = function() {};
A.prototype = {
_instance_var: 0,
initialize: function(v) { this._instance_var = v; },
x: function() { alert(this._instance_var); }
};
EDIT Didn't mean to imply that the scope of the variable was private, I was just trying to illustrate how I define my classes in javascript. Variable name has been changed to reflect this.
As discussed in other answers, it's really a performance consideration because the function in the prototype is shared with all of the instantiations - rather than the function being created for each instantiation.
I put together a jsperf to show this. There is a dramatic difference in the time it takes to instantiate the class, although it is really only relevant if you are making many instances.
http://jsperf.com/functions-in-constructor-vs-prototype
Think about statically typed language, things on prototype are static and things on this are instance related.
When you use prototype, the function will only be loaded only once into memory (independently on the amount of objects you create) and you can override the function whenever you want.

Performance disadvantages of using this javascript pattern?

I am trying to duplicate private prototype methods in javascript and found this bit of code, but I do not fully understand how it implements itself.
The code is...
var Foo = function () {};
Foo.prototype = (function () {
// Private attributes
var somePrivateAttribute = 'Hello world';
// Private methods
function somePrivateMethod(val) {
alert(val);
}
// Public attributes and methods
return {
somePublicMethod: function () {
somePrivateMethod(somePrivateAttribute);
}
};
})();
What I do not understand is when this object is initiated new Foo() does that create a new object for the prototype every time as it is in a self executing function? Could anyone please explain the benefits and disadvantages of using this for a project of mine.
How would this type of pattern work for objects that could have 500 instances or just 1. I am trying to understand and would greatly appreciate if anyone could enlighten me?
Is there any performance benefits to using this method?
What I do not understand is when this object is initiated new Foo() does that create a new object for the prototype every time as it is in a self executing function?
No. That's the point of prototypes, they are not duplicated - every of your Foo instances will inherit from the same Foo.prototype object.
Could anyone please explain the benefits and disadvantages of using this for a project of mine.
It looks like you're having a little misunderstanding, regarding the naming: Those "private" things are no "attributes" or "methods", as they have absolutely nothing to do with instance objects. They are just local variables which are only accessible from the functions you assign to the prototype object.
You might want to read How do JavaScript closures work? and their use in IIFEs.
How would this type of pattern work for objects that could have 500 instances or just 1.
It's perfectly fine. However, using the constructor pattern for just 1 instance might be a little odd - singletons can be defined in a simpler way.
If you want to read more on that pattern, search for revealing prototype pattern - it's the revealing module pattern applied to create prototype objects.
I haven't seen that pattern before, using a module (that's what it's called) for the prototype... but here's what I'd recommend; it's similar but the module encloses everything:
var Foo = (function FooClass() {
var privateVar = 'Hello world';
function privateFunc() {
...
}
function Foo() {}
Foo.prototype = {
publicMethod: function() {
...
}
};
return Foo;
}());
var foo = new Foo();
What I do not understand is when this object is initiated new Foo()
does that create a new object for the prototype every time as it is in
a self executing function?
The self executing function runs once, returns the object, and it won't run again; it keeps a reference to the variables via closure.
What I do not understand is when this object is initiated new Foo() does that create a new object for the prototype every time as it is in a self executing function?
No, the assignment to Foo's prototype only happens once. Nothing wrong with it at all, just a little unconventional.
You could also write it like this (also unconventional, but accomplishes the same thing):
var Foo = function () {};
Foo.prototype = new function() {
// Private attributes
var somePrivateAttribute = 'Hello world';
// Private methods
function somePrivateMethod(val) {
alert(val);
}
// Public attributes and methods
this.constructor = Foo;
this.somePublicMethod = function () {
somePrivateMethod(somePrivateAttribute);
}
};
As Bergi pointed out, this adds an extra object to the prototype chain; if you want to avoid that, you can write it like this instead:
var Foo = function () {};
(function() {
// Private attributes
var somePrivateAttribute = 'Hello world';
// Private methods
function somePrivateMethod(val) {
alert(val);
}
// Public attributes and methods
this.somePublicMethod = function () {
somePrivateMethod(somePrivateAttribute);
}
}).call(Foo.prototype)
By modifying the prototype instead of replacing it, the constructor property also stays intact.

improve javascript prototypal inheritance

I'm using a classical javascript prototypal inheritance, like this:
function Foo() {}
Naknek.prototype = {
//Do something
};
var Foo = window.Foo = new Foo();
I want to know how I can improve this and why I can't use this model:
var Foo = window.Foo = new function() {
};
Foo.prototype = {
//Do something
};
Why this code doesn't work? In my head this is more logical than the classical prototypal inheritance.
Your var Foo = window.Foo = new Foo(); example is...odd. First you declare a function Foo, then you assign something to its prototype (I assume you meant to replace Nannak with Foo in your second line), and then you overwrite the reference to the function by assigning to it.
Standard Javascript prototypical inheritance looks like this:
// Create a constructor `Foo`
function Foo() {
}
// Give the prototype for `Foo` objects a function bound to the
// property `bar`
Foo.prototype.bar = function() {
}
// Only do this bit if you're doing this within a function and you want
// to make `Foo` available as a property of `window` (e.g., effectively
// a global variable. If this code is already at global scope, there's
// no need to do this.
window.Foo = Foo;
Here's how you'd use Foo:
// Create a `Foo` and use its `bar`
var f = new Foo();
f.bar();
A couple of other notes:
Never use new function() { ... } unless you really, really know what you're doing. It creates an object initialized by the function, it does not create a new function. There's almost never any need for the similar (but totally different) new Function(...), either, again except for some advanced edge cases.
The function bound to bar in my example above is anonymous (the property it's bound to has a name, but the function does not). I'm not a fan of anonymous functions, but I didn't want to clutter the example above. More here.
If you use var at global scope (it wasn't clear from your question whether you were), it creates a property on the window object. You'd only need to do var Foo = window.Foo = ... if you're within a function and want to both create a propety of window and create a local variable called Foo. (Which maybe you meant to do! :-) )
The Foo.prototype = { .. }; way of assigning prototypical properties works, but it's not generally a good idea (again, unless you're pretty up-to-speed on this stuff). When you do that, you're completely replacing the prototype object used when you call the function via new, whereas if you just add properties to it (Foo.prototype.bar = ...), you're just augmenting the prototype, not replacing it. Replacing the prototype is perfectly valid, but there are some non-obvious things that can happen if you're unwary. :-)
You are using the new operator on the function expression, that causes a newly created object to be assigned to window.Foo.
That new object is not a function, therefore it doesn't have a prototype property and it cannot be invoked.
"doesn't work" is vague, but I think the root cause is new function() { (new doesn't belong here)
You probably want:
var Foo = window.Foo = function() {
};
Foo.prototype = {
bar: 'baz'
};
alert(new Foo().bar); // baz

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