Develop Reference

Difference between Factory function and Constructor [duplicate]

Can someone clarify the difference between a constructor function and a factory function in Javascript.
When to use one instead of the other?

The basic difference is that a constructor function is used with the new keyword (which causes JavaScript to automatically create a new object, set this within the function to that object, and return the object):
var objFromConstructor = new ConstructorFunction();
A factory function is called like a "regular" function:
var objFromFactory = factoryFunction();
But for it to be considered a "factory" it would need to return a new instance of some object: you wouldn't call it a "factory" function if it just returned a boolean or something. This does not happen automatically like with new, but it does allow more flexibility for some cases.
In a really simple example the functions referenced above might look something like this:
function ConstructorFunction() {
this.someProp1 = "1";
this.someProp2 = "2";
}
ConstructorFunction.prototype.someMethod = function() { /* whatever */ };
function factoryFunction() {
var obj = {
someProp1 : "1",
someProp2 : "2",
someMethod: function() { /* whatever */ }
};
// other code to manipulate obj in some way here
return obj;
}
Of course you can make factory functions much more complicated than that simple example.
One advantage to factory functions is when the object to be returned could be of several different types depending on some parameter.

Benefits of using constructors
Most books teach you to use constructors and new
this refers to the new object
Some people like the way var myFoo = new Foo(); reads.
Drawbacks
Details of instantiation get leaked into the calling API (via the new requirement), so all callers are tightly coupled to the constructor implementation. If you ever need the additional flexibility of the factory, you'll have to refactor all callers (admittedly the exceptional case, rather than the rule).
Forgetting new is such a common bug, you should strongly consider adding a boilerplate check to ensure that the constructor is called correctly ( if (!(this instanceof Foo)) { return new Foo() } ). EDIT: Since ES6 (ES2015) you can't forget new with a class constructor, or the constructor will throw an error.
If you do the instanceof check, it leaves ambiguity as to whether or not new is required. In my opinion, it shouldn't be. You've effectively short circuited the new requirement, which means you could erase drawback #1. But then you've just got a factory function in all but name, with additional boilerplate, a capital letter, and less flexible this context.
Constructors break the Open / Closed Principle
But my main concern is that it violates the open/closed principle. You start out exporting a constructor, users start using the constructor, then down the road you realize you need the flexibility of a factory, instead (for instance, to switch the implementation to use object pools, or to instantiate across execution contexts, or to have more inheritance flexibility using prototypal OO).
You're stuck, though. You can't make the change without breaking all the code that calls your constructor with new. You can't switch to using object pools for performance gains, for instance.
Also, using constructors gives you a deceptive instanceof that doesn't work across execution contexts, and doesn't work if your constructor prototype gets swapped out. It will also fail if you start out returning this from your constructor, and then switch to exporting an arbitrary object, which you'd have to do to enable factory-like behavior in your constructor.
Benefits of using factories
Less code - no boilerplate required.
You can return any arbitrary object, and use any arbitrary prototype - giving you more flexibility to create various types of objects which implement the same API. For example, a media player that can create instances of both HTML5 and flash players, or an event library which can emit DOM events or web socket events. Factories can also instantiate objects across execution contexts, take advantage of object pools, and allow for more flexible prototypal inheritance models.
You'd never have a need to convert from a factory to a constructor, so refactoring will never be an issue.
No ambiguity about using new. Don't. (It will make this behave badly, see next point).
this behaves as it normally would - so you can use it to access the parent object (for example, inside player.create(), this refers to player, just like any other method invocation would. call and apply also reassign this, as expected. If you store prototypes on the parent object, that can be a great way to dynamically swap out functionality, and enable very flexible polymorphism for your object instantiation.
No ambiguity about whether or not to capitalize. Don't. Lint tools will complain, and then you'll be tempted to try to use new, and then you'll undo the benefit described above.
Some people like the way var myFoo = foo(); or var myFoo = foo.create(); reads.
Drawbacks
new doesn't behave as expected (see above). Solution: don't use it.
this doesn't refer to the new object (instead, if the constructor is invoked with dot notation or square bracket notation, e.g. foo.bar() - this refers to foo - just like every other JavaScript method -- see benefits).

A constructor returns an instance of the class you call it on. A factory function can return anything. You would use a factory function when you need to return arbitrary values or when a class has a large setup process.

A Constructor function example
function User(name) {
this.name = name;
this.isAdmin = false;
}
let user = new User("Jack");
new creates an object prototyped on User.prototype and calls User with the created object as its this value.
new treats an argument expression for its operand as optional:
let user = new User;
would cause new to call User with no arguments.
new returns the object it created, unless the constructor returns an object value, which is returned instead. This is an edge case which for the most part can be ignored.
Pros and Cons
Objects created by constructor functions inherit properties from the constructor's prototype property, and return true using the instanceOf operator on the constructor function.
The above behaviors can fail if you dynamically change the value of the constructor's prototype property after having already used the constructor. Doing so is rare, and it can't be changed if the constructor were created using the class keyword.
Constructor functions can be extended using the extends keyword.
Constructor functions can't return null as an error value. Since it's not an object data type, it is ignored by new.
A Factory function example
function User(name, age) {
return {
name,
age,
}
};
let user = User("Tom", 23);
Here the factory function is called without new. The function is entirely responsible for the direct or indirect use if its arguments and the type of object it returns. In this example it returns a simple [Object object] with some properties set from arguments.
Pros and Cons
Easily hides the implementation complexities of object creation from the caller. This is particularly useful for native code functions in a browser.
The factory function need not always return objects of the same type, and could even return null as an error indicator.
In simple cases, factory functions can be simple in structure and meaning.
Objects returned do not generally inherit from the factory function's prototype property, and return false from instanceOf factoryFunction.
The factory function can't be safely extended using the extends keyword because extended objects would inherit from the factory functions prototype property instead of from the prototype property of the constructor used by the factory function.

Factories are "always" better. When using object orientated languages then
decide on the contract (the methods and what they will do)
Create interfaces that expose those methods (in javascript you don't have interfaces so you need to come up with some way of checking the implementation)
Create a factory that returns an implementation of each interface required.
The implementations (the actual objects created with new) are not exposed to the factory user/consumer. This means that the factory developer can expand and create new implementations as long as he/she doesn't break the contract...and it allows for the factory consumer to just benefit from the new API without having to change their code...if they used new and a "new" implementation comes along then they have to go and change every line which uses "new" to use the "new" implementation...with the factory their code doesn't change...
Factories - better than all anything else - the spring framework is completely built around this idea.

Factories are a layer of abstraction, and like all abstractions they have a.cost in complexity. When encountering a factory based API figuring out what the factory is for a given API can be challenging for the API consumer. With constructors discoverability is trivial.
When deciding between ctors and factories you need to decide if the complexity is justified by the benefit.
Worth noting that Javascript constructors can be arbitrary factories by returning something other than this or undefined. So in js you can get the best of both worlds - discoverable API and object pooling/caching.

I think the factory function is superior to the constructor function. Using new with the constructor function, we are binding our code to one specific way of creating an object, while with a factory, we are free so we can create more different instances without binding ourselves. Let's say we have this class:
const file = new CreateFile(name)
If we want to refactor CreateFile class, creating subclasses for the file format our server supports, we can write an elegan factory function:
function CreateFile(name) {
if (name.match(/\.pdf$/)) {
return new FilePdf(name);
} else if (name.match(/\.txt$/)) {
return new FileTxt(name);
} else if (name.match(/\.md$/)) {
return new FileMd(name);
} else {
throw new Error("Not supprted file type");
}
}
with factory functions, we can implement private variables, hide the information from the users which is called encapsulation.
function createPerson(name) {
const privateInfo = {};
// we create person object
const person = {
setName(name) {
if (!name) {
throw new Error("A person must have a name");
}
privateInfo.name = name;
},
getName() {
return privateInfo.name;
},
};
person.setName(name);
return person;
}

For the differences, Eric Elliott clarified very well,
But for the second question:
When to use one instead of the other?
If you are coming from the object-oriented background, Constructor function looks more natural to you.
this way you shouldn't forget to use new keyword.

Usecase for JavaScript prototype [duplicate]

I'd like to understand when it is appropriate to use prototype methods in js. Should they always be used? Or are there cases where using them is not preferred and/or incurs a performance penalty?
In searching around this site on common methods for namespacing in js, it seems that most use a non-prototype based implementation: simply using an object or a function object to encapsulate a namespace.
Coming from a class-based language, it's hard not to try and draw parallels and think that prototypes are like "classes" and the namespace implementations I mentioned are like static methods.

Prototypes are an optimisation.
A great example of using them well is the jQuery library. Every time you obtain a jQuery object by using $('.someClass'), that object has dozens of "methods". The library could achieve that by returning an object:
return {
show: function() { ... },
hide: function() { ... },
css: function() { ... },
animate: function() { ... },
// etc...
};
But that would mean that every jQuery object in memory would have dozens of named slots containing the same methods, over and over.
Instead, those methods are defined on a prototype and all jQuery objects "inherit" that prototype so as to gain all those methods at very little runtime cost.
One vitally important part of how jQuery gets it right is that this is hidden from the programmer. It's treated purely an optimisation, not as something that you have to worry about when using the library.
The problem with JavaScript is that naked constructor functions require the caller to remember to prefix them with new or otherwise they typically don't work. There is no good reason for this. jQuery gets it right by hiding that nonsense behind an ordinary function, $, so you don't have to care how the objects are implemented.
So that you can conveniently create an object with a specified prototype, ECMAScript 5 includes a standard function Object.create. A greatly simplified version of it would look like this:
Object.create = function(prototype) {
var Type = function () {};
Type.prototype = prototype;
return new Type();
};
It just takes care of the pain of writing a constructor function and then calling it with new.
When would you avoid prototypes?
A useful comparison is with popular OO languages such as Java and C#. These support two kinds of inheritance:
interface inheritance, where you implement an interface such that the class provides its own unique implementation for every member of the interface.
implementation inheritance, where you extend a class that provides default implementations of some methods.
In JavaScript, prototypical inheritance is a kind of implementation inheritance. So in those situations where (in C# or Java) you would have derived from a base class to gain default behaviour, which you then make small modifications to via overrides, then in JavaScript, prototypical inheritance makes sense.
However, if you're in a situation where you would have used interfaces in C# or Java, then you don't need any particular language feature in JavaScript. There is no need to explicitly declare something that represents the interface, and no need to mark objects as "implementing" that interface:
var duck = {
quack: function() { ... }
};
duck.quack(); // we're satisfied it's a duck!
In other words, if each "type" of object has its own definitions of the "methods", then there is no value in inheriting from a prototype. After that, it depends on how many instances you allocate of each type. But in many modular designs, there is only one instance of a given type.
And in fact, it has been suggested by many people that implementation inheritance is evil. That is, if there are some common operations for a type, then maybe it's clearer if they are not put into a base/super class, but are instead just exposed as ordinary functions in some module, to which you pass the object(s) you want them to operate on.

You should use prototypes if you wish to declare a "non-static" method of the object.
var myObject = function () {
};
myObject.prototype.getA = function (){
alert("A");
};
myObject.getB = function (){
alert("B");
};
myObject.getB(); // This works fine
myObject.getA(); // Error!
var myPrototypeCopy = new myObject();
myPrototypeCopy.getA(); // This works, too.

One reason to use the built-in prototype object is if you'll be duplicating an object multiple times that will share common functionality. By attaching methods to the prototype, you can save on duplicating methods being created per each new instance. But when you attach a method to the prototype, all instances will have access to those methods.
Say you have a base Car() class/object.
function Car() {
// do some car stuff
}
then you create multiple Car() instances.
var volvo = new Car(),
saab = new Car();
Now, you know each car will need to drive, turn on, etc. Instead of attaching a method directly to the Car() class (which takes up memory per each instance created), you can attach the methods to the prototype instead (creating the methods only once), therefore giving access to those methods to both the new volvo and saab.
// just mapping for less typing
Car.fn = Car.prototype;
Car.fn.drive = function () {
console.log("they see me rollin'");
};
Car.fn.honk = function () {
console.log("HONK!!!");
}
volvo.honk();
// => HONK!!!
saab.drive();
// => they see me rollin'

Put functions on a prototype object when you're going to create lots of copies of a particular kind of object and they all need to share common behaviors. By doing so, you'll save some memory by having just one copy of each function, but that's only the simplest benefit.
Changing methods on prototype objects, or adding methods, instantly changes the nature of all the instances of the corresponding type(s).
Now exactly why you'd do all these things is mostly a function of your own application design, and the kinds of things you need to do in client-side code. (A whole different story would be code inside a server; much easier to imagine doing more large-scale "OO" code there.)

If i explain in class based term then Person is class, walk() is Prototype method. So walk() will have its existence only after you instantiate new object with this.
So if you want to create the copies of object like Person u can create many users Prototype is good solution as it saves memory by sharing/inheriting same copy of function for each of the object in memory.
Whereas static is not that great help in such scenario.
function Person(){
this.name = "anonymous";
}
// its instance method and can access objects data data
Person.prototype.walk = function(){
alert("person has started walking.");
}
// its like static method
Person.ProcessPerson = function(Person p){
alert("Persons name is = " + p.name);
}
var userOne = new Person();
var userTwo = new Person();
//Call instance methods
userOne.walk();
//Call static methods
Person.ProcessPerson(userTwo);
So with this its more like instance method.
The object's approach is like Static methods.
https://developer.mozilla.org/en/Introduction_to_Object-Oriented_JavaScript

Just including a video link here for reference, when not to use prototypes: https://youtu.be/JCXZhe6KsxQ?t=2m30s
Here is Ben Lesh's talk from NGConf, why rxjs removed patching prototype (chainable functions) in favor of pipeable functions.

What object is being returned by .responseXML()?

I am trying to create a prototype/class that inherits from the object that is returned by a specific function. But I dont know what that object's name is?
For the instance var xhr = XMLHttpRequest(); what object is returned by xhr.responseXML;? Is it XMLDocument? Or maybe XMLDOM?
Also if I create the object var xmlDoc = new ActiveXObject("Microsoft.XMLDOM"); and call xmlDoc.load("xmlFile.xml" ); will it return the same type of object as xhr.responseXML;?
I am trying to do the following:
function XMLHandler()
{
this.xmlFile = "defaultXML.xml";
}
// Make XMLHandler inherit from Javascript object
XMLHandler.prototype = new XMLDocument();
XMLHandler.prototype.constructor = XMLDocument;

You shouldn't include unrelated questions in the same post. To address your second question about declaring a function within a "class":
Your first syntax, declaring it within the constructor, will pointlessesly overwrite myClass.prototype.publicFunct to point to a newly created function every time the constructor is called. That is, you will be creating an extra copy of the function with each call to the constructor, with myClass.prototype.publicFunct always pointing to the most recently created copy - or undefined until the constructor has been called at least once. Don't do this.
Your second option is simply invalid syntax in JavaScript.
Your third syntax is prefered. This will add the function to the prototype once.
Remember: JavaScript doesn't have classes as such, though you can sort of simulate them if you think it is worth the bother.

I can tell by your recent questions that you are thinking like Java, which this is not.
First question:
responseXML is different for each browser. Firefox gives a nsIDOMDocument, IE gives an IXMLDOMDocument and Webkit browsers depend on the responseType setting but will probably be Document. Since you cannot predict what it will be stop trying to extend it. In most cases the type isn't made available by the browser's API so javascript cannot extend it anyway.
Moreover, since JavaScript's inheritance is not class based you are forced into doing this:
XMLHandler.prototype = new XMLDocument();
...which simply does not work for your purpose. Any instance of XMLHandler will be built on an unrelated, empty document and not the document returned by responseXML. You have to use a wrapper here.
Second Question:
Of your 3 methods the first is equivalent to the last but more wasteful because it repeatedly sets the same function to the same prototype. The second is nonsensical, the syntax is broken. These are your real options:
// Instance method, every instance is given a copy of the function upon "new"
function MyClass()
{
this.publicFunct = function()
{
alert("public function");
};
}
// Prototypal method, only one copy of the function shared by all instances
function MyClass()
{
}
MyClass.prototype.publicFunct = function()
{
alert("public function");
};
// Shorthand method, same as prototypal but handy for several members at once
// It prevents MyClass from being a descendent of another type
function MyClass()
{
}
MyClass.prototype = {
// A colon is only acceptable in object notation
publicFunct: function()
{
alert("public function");
}
};
I would go with a prototypal method for efficiency unless you need to selectively add functions to a class. Your use of "public function" (also "class") seems like another symptom of an OOP background, there aren't any private functions in JavaScript so "public" has no place, all member functions are public. If at some point you do need a private function you can fake the effect with a closure.
(function() {
// Assignments are mostly global
MyClass = function() {};
MyClass.prototype.publicFunct = function()
{
privateFunct();
};
// These statements affect local scope
var foo = 'bar';
function privateFunct()
{
alert("public function");
}
})(); // These extra brackets cause the contents to be executed immediately
Having said that it is rare to need private functions and all JavaScript is visible anyway so it's not secret really. The above could be thwarted like this:
thief = {};
MyClass.prototype.publicFunct.call(thief);
// privateFunct is called by publicFunct in the context of the thief
You might as well accept that functions are public. You can go a step further and give up on classes altogether. Objects are just objects that happen to have some functions and those functions can even be shared with completely different objects.

Is it customary JS to write getters for simple properties?

Is this code idomatic, or should I just acess the properties directly?
self.isAlive = function() {
return self.alive;
}

It all depends on implementation.
If you feel the property is always going to be the value, accessing the property is adequate. If you feel like there's going to be some logic that determines returning a "properly formatted" value, I would use a method (or if the value could possibly be dynamic based on other factors, a method is a good idea).
To go a step further, other languages like C# have properties making it safe to expose values, but not necessarily alter them. JS doesn't have such a method of protecting it, so often get<Var> is used to expose the property, but in a read-only fashion.
It's all about how you would like the data, while also trying to anticipate what the worst-case scenario could be if you expose the property directly vs. through a method.

It seems that you are speaking of properties in the classical OOP sense, i.e., properties of classes. In the current version of Javascript, simulating this behavior is not completely clean. Consider the following example:
var Widget = function() {
var private = 'private';
}
'Widget' is a constructor function, and 'private' will be instantiated in the scope of just the constructor function. That means there is only one way to get access to 'private' from outside the constructor function scope:
var Widget = function() {
var private = 'private';
this.getPrivate = function() {
return private;
}
}
This will hide the private variable for each instance of Widget, but unfortunately, 'getPrivate' must now be added to each instance of Widget. You can see this occurring in the constructor function by the statement this.getPrivate = .... Every time you make an instance of Widget using var widget1 = new Widget(), that specific instance will have a new function, 'getPrivate', added to it.
The normal way to make reusable components in Javascript is to use its implementation of Prototypical Inheritance. You assign a prototype object to the constructor function that will have shared methods and properties across all instances of the component. I will provide an example below to explain, but there are caveats to prototypical inheritance that you should become aware of if you are doing object-oriented javascript. A great resource.
Here's an example of how this might come in to play for your example:
var Widget = function() {
this.alive = true;
}
Widget.prototype.isAlive = function() {
return this.alive;
}
var widget1 = new Widget();
widget1.isAlive(); // returns true
The problem here is that the property alive is being added to the object, and thus, it is publicly available, i.e., any user of your Widget can simply look through the DOM in their favorite variation of FireBug and see (as well as freely modify) this property.
Consider a variation where you attempt to hide alive to external users. Because alive is a property of Widget instances, you would want to include the code to hide it in the constructor, as shown below:
var Widget = function() {
var alive = true;
}
However, as stated at the beginning of this post, alive in this case is only exposed to the constructor function's scope; therefore, it would not be accessible to methods on the prototype. The reason for this is that each of those methods do not have the same scope as the constructor function, since Javascript only has function scope.
In conclusion, if you're trying to hide data in a reusable component, javascript does not provide a clean way to do it. If you don't mind having new memory allocated on each component instance for the same methods, then you can use my second code example in this post. However, if you prefer to have your methods allocated in memory only once, then you will need to expose properties on the object instance, e.g., this.alive = true.

That's subjective. Use of getters and setters is very discussed. Some people (like me) say you should always use getters and setters to follow the encapsulation principle, while others say that you shouldn't since your application would become unmaintainable.

This is good practice if you don't want outside js code to modify your property. If this is the desired effect you might consider assigning that property using var instead of making it a property of that object. That way your code within the object can modify it but it can only be accessed through your getter
var alive = false;
function isAlive(){ return alive;}

You can use:
function isAlive(){
return arguments.callee.alive;
}
isAlive.alive=true;
This appears to be ok still in strict mode.
Another way to accomplish this is
function isAlive(){
return isAlive.alive;
}
isAlive.alive=true;
But the problem is that someone can use the name as a string, or rename the function etc... and you can loose the status.

Javascript when to use prototypes

I'd like to understand when it is appropriate to use prototype methods in js. Should they always be used? Or are there cases where using them is not preferred and/or incurs a performance penalty?
In searching around this site on common methods for namespacing in js, it seems that most use a non-prototype based implementation: simply using an object or a function object to encapsulate a namespace.
Coming from a class-based language, it's hard not to try and draw parallels and think that prototypes are like "classes" and the namespace implementations I mentioned are like static methods.

Prototypes are an optimisation.
A great example of using them well is the jQuery library. Every time you obtain a jQuery object by using $('.someClass'), that object has dozens of "methods". The library could achieve that by returning an object:
return {
show: function() { ... },
hide: function() { ... },
css: function() { ... },
animate: function() { ... },
// etc...
};
But that would mean that every jQuery object in memory would have dozens of named slots containing the same methods, over and over.
Instead, those methods are defined on a prototype and all jQuery objects "inherit" that prototype so as to gain all those methods at very little runtime cost.
One vitally important part of how jQuery gets it right is that this is hidden from the programmer. It's treated purely an optimisation, not as something that you have to worry about when using the library.
The problem with JavaScript is that naked constructor functions require the caller to remember to prefix them with new or otherwise they typically don't work. There is no good reason for this. jQuery gets it right by hiding that nonsense behind an ordinary function, $, so you don't have to care how the objects are implemented.
So that you can conveniently create an object with a specified prototype, ECMAScript 5 includes a standard function Object.create. A greatly simplified version of it would look like this:
Object.create = function(prototype) {
var Type = function () {};
Type.prototype = prototype;
return new Type();
};
It just takes care of the pain of writing a constructor function and then calling it with new.
When would you avoid prototypes?
A useful comparison is with popular OO languages such as Java and C#. These support two kinds of inheritance:
interface inheritance, where you implement an interface such that the class provides its own unique implementation for every member of the interface.
implementation inheritance, where you extend a class that provides default implementations of some methods.
In JavaScript, prototypical inheritance is a kind of implementation inheritance. So in those situations where (in C# or Java) you would have derived from a base class to gain default behaviour, which you then make small modifications to via overrides, then in JavaScript, prototypical inheritance makes sense.
However, if you're in a situation where you would have used interfaces in C# or Java, then you don't need any particular language feature in JavaScript. There is no need to explicitly declare something that represents the interface, and no need to mark objects as "implementing" that interface:
var duck = {
quack: function() { ... }
};
duck.quack(); // we're satisfied it's a duck!
In other words, if each "type" of object has its own definitions of the "methods", then there is no value in inheriting from a prototype. After that, it depends on how many instances you allocate of each type. But in many modular designs, there is only one instance of a given type.
And in fact, it has been suggested by many people that implementation inheritance is evil. That is, if there are some common operations for a type, then maybe it's clearer if they are not put into a base/super class, but are instead just exposed as ordinary functions in some module, to which you pass the object(s) you want them to operate on.

You should use prototypes if you wish to declare a "non-static" method of the object.
var myObject = function () {
};
myObject.prototype.getA = function (){
alert("A");
};
myObject.getB = function (){
alert("B");
};
myObject.getB(); // This works fine
myObject.getA(); // Error!
var myPrototypeCopy = new myObject();
myPrototypeCopy.getA(); // This works, too.

One reason to use the built-in prototype object is if you'll be duplicating an object multiple times that will share common functionality. By attaching methods to the prototype, you can save on duplicating methods being created per each new instance. But when you attach a method to the prototype, all instances will have access to those methods.
Say you have a base Car() class/object.
function Car() {
// do some car stuff
}
then you create multiple Car() instances.
var volvo = new Car(),
saab = new Car();
Now, you know each car will need to drive, turn on, etc. Instead of attaching a method directly to the Car() class (which takes up memory per each instance created), you can attach the methods to the prototype instead (creating the methods only once), therefore giving access to those methods to both the new volvo and saab.
// just mapping for less typing
Car.fn = Car.prototype;
Car.fn.drive = function () {
console.log("they see me rollin'");
};
Car.fn.honk = function () {
console.log("HONK!!!");
}
volvo.honk();
// => HONK!!!
saab.drive();
// => they see me rollin'

Put functions on a prototype object when you're going to create lots of copies of a particular kind of object and they all need to share common behaviors. By doing so, you'll save some memory by having just one copy of each function, but that's only the simplest benefit.
Changing methods on prototype objects, or adding methods, instantly changes the nature of all the instances of the corresponding type(s).
Now exactly why you'd do all these things is mostly a function of your own application design, and the kinds of things you need to do in client-side code. (A whole different story would be code inside a server; much easier to imagine doing more large-scale "OO" code there.)

If i explain in class based term then Person is class, walk() is Prototype method. So walk() will have its existence only after you instantiate new object with this.
So if you want to create the copies of object like Person u can create many users Prototype is good solution as it saves memory by sharing/inheriting same copy of function for each of the object in memory.
Whereas static is not that great help in such scenario.
function Person(){
this.name = "anonymous";
}
// its instance method and can access objects data data
Person.prototype.walk = function(){
alert("person has started walking.");
}
// its like static method
Person.ProcessPerson = function(Person p){
alert("Persons name is = " + p.name);
}
var userOne = new Person();
var userTwo = new Person();
//Call instance methods
userOne.walk();
//Call static methods
Person.ProcessPerson(userTwo);
So with this its more like instance method.
The object's approach is like Static methods.
https://developer.mozilla.org/en/Introduction_to_Object-Oriented_JavaScript

Just including a video link here for reference, when not to use prototypes: https://youtu.be/JCXZhe6KsxQ?t=2m30s
Here is Ben Lesh's talk from NGConf, why rxjs removed patching prototype (chainable functions) in favor of pipeable functions.