How does javascript prototype work

JavaScript internally makes an object, an empty hash. Then it gives that object to the constructor, where the constructor can do whatever it wants because this inside of that constructor is the object that was just created. Finally, it gives you that object if you haven't used the return statement in your function, or if you've put a return undefined; at the end of your function body. So when JavaScript goes to look up a property on an object, the first thing it does is it looks it up on that object.

In other words, when you have a prototype property on a function and you call a new on that, after JavaScript finishes looking at that newly created object for properties, it will go look at the function's. Thus, it is possible that this object has its own internal prototype, and so on. Mobile App Development.

Programming Languages. Get insights on scaling, management, and product development for founders and engineering managers. Read programming tutorials, share your knowledge, and become better developers together. Hot Topics. Mehran Hatami Follow. Javascript Developer and Front End Engineer. Published May 04, Last updated Feb 09, Attach the function to an object as its property - The easiest way to do this is by modifying the empty person object, like: person.

Person 'George' ; person. I'll explain this a bit later in the summary, but what's important is we can get a similar result by doing: person. So what could be the better practice now: person. So to make sure this works, you can do: console. This story continues until you reach the prototypical object that provides methods that are accessible on all objects like.

The [[Prototype]] property is part of what forms the [[Prototype]] chain. This chain of [[Prototype]] objects is what is examined when, for example, [[Get]] or [[Set]] operations are performed on an object:.

You can easily examine this:. One of the most important. This prototype holds the prototypical object that all [[Prototype]] chains contain. On it, all the available methods for new objects are defined:. Now, since. When you don't make any assignments to Function. This is automatically performed anytime you create a new function.

This way, any time you do new Bar; the prototype chain is set up for you, you get everything defined on Bar. When you do make assignments to Function. It's like an insertion in a singly linked list.

This basically alters the [[Prototype]] chain allowing properties that are defined on the object assigned to Function. Let me tell you my understanding of prototypes. I am not going to compare the inheritance here with other languages. I wish people would stop comparing languages, and just understand the language as itself.

Understanding prototypes and prototypal inheritance is so simple, as I will show you below. Prototype is like a model, based on which you create a product. The crucial point to understand is that when you create an object using another object as it's prototype, the link between the prototype and the product is ever-lasting. For instance:. Every object contains an internal property called the [[prototype]], which can be accessed by the Object.

Hence when you do Object. Such a linking of objects using the prototype property is called prototypal inheritance. There, it is so simple, agree? Another attempt to explain JavaScript prototype-based inheritance with better pictures. Now, suppose we create instances of this keyValueStore object. Even though get , set , delete , getLength will do the exact same thing for each of these instances, every instance has its own copy of this function.

Now, imagine if you could have just a single get , set , delete and getLength copy, and each instance would reference that same function. This would be better for performance and require less memory.

That's where prototypes come in. A prototype is a "blueprint" of properties that is inherited but not copied by instances. So this means that it exists only once in memory for all instances of an object and is shared by all of those instances. What this means, is that all of the instances now share these four methods instead of each having their own copy. Javascript has a mechanism when looking up properties on Objects which is called 'prototypal inheritance' , here is what is basically does:.

I always like analogies when it comes to understand this type of stuff. In fact with prototypes, there really is no inheritance, so the name in and of itself misleading, it's more a type of 'delegation'.

You're in high-school, and you're in class and have a quiz that's due today, but you don't have a pen to fill out your answers. You're sitting next to your friend Finnius, who might have a pen. You ask, and he looks around his desk unsuccessfully, but instead of saying "I don't have a pen", he's a nice friend he checks with his other friend Derp if he has a pen. Derp does indeed have a spare pen and passes it back to Finnius, who passes it over to you to complete your quiz.

Derp has entrusted the pen to Finnius, who has delegated the pen to you for use. What is important here is that Derp does not give the pen to you, as you don't have a direct relationship with him. This, is a simplified example of how prototypes work, where a tree of data is searched for the thing you're looking for.

It's just that you already have an object with Object. It's important to understand that there is a distinction between an object's prototype which is available via Object. The former is the property on each instance, and the latter is the property on the constructor.

That is, Object. The Prototype creates new object by cloning existing object. So really when we think about prototype we can really think cloning or making a copy of something instead of making it up. If you want to understand the concept of prototype and prototype based inheritance from the basics, check the official MDN docs, they explain it pretty well. When it comes to inheritance, JavaScript only has one construct: objects.

Each object has a private property which holds a link to another object called its prototype. That prototype object has a prototype of its own, and so on until an object is reached with null as its prototype.

By definition, null has no prototype, and acts as the final link in this prototype chain. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow.

Learn more. How does JavaScript. Ask Question. Asked 12 years, 8 months ago. Active 1 year, 1 month ago. Viewed k times. Improve this question. Community Bot 1 1 1 silver badge. John Leidegren John Leidegren 57k 18 18 gold badges silver badges bronze badges. John Resig has a few slides on function prototypes that were helpful to me when looking into the subject you can also make changes to the code and see what happens Great reference material, for purposes of keeping this question informative perhaps place some of the comments from John's site on your answer in case his site is changes in a way that your link is no longer available.

Starting from there was really helpful, and I feel like I understand prototypes correctly. Do we really need a functional object for applying prototype?

This might help you: webdeveasy. Show 5 more comments. Active Oldest Votes. Improve this answer. Tushar Gupta - curioustushar I think the answers on stackoverflow are not only interesting to the original poster, but also to a big community of other people lurking or coming from searches. And I've been one of them and I had benefit from old posts. I think I could contribute to the other answers adding some code examples. About your question: if you leave out the new, it doesn't work.

The easiest way is experiment with firebug and see what happens. So there is no Object yet, only the anonymous constructor function is assigned to Person.

This is a very good explanation: helephant. The only reason it works is because he did the exact same thing setting the name in both the child and parent constructor.

For a more in depth explanation on common mistakes made when attempting inheritance in JavaScript and a final solution , please see: this stack overflow post — Aaren Cordova. About the Customer. Add a comment. An object's [[Prototype]] is initially set during object creation. Ben Aston Christoph Christoph k 36 36 gold badges silver badges bronze badges.

So JavaScript has 3 different ways to do that: a. We can attach them to an object, as its properties - The easiest way to do this is modifying the empty person object, like: person. Person "George" ; person. So we could get the similar result by doing: person. So what could be the better practice now: person. So to make sure you can do: console. JavaScript has another way to provide the function with this , which is using call or apply to invoke the function.

Pynchia 9, 5 5 gold badges 29 29 silver badges 41 41 bronze badges. Mehran Hatami Mehran Hatami It can be retrieved via the ES5 Object. Intuitively, classical inheritance should affect property lookup. Lookup order is: obj properties added with obj.

This is the so-called prototype chain. On the second line, when we do: c. Y , JavaScript automatically sets this to equal X inside the Y function call! The exact same logic also explains d. Some of them may not be achievable without the class keyword TODO check which : [[FunctionKind]]:"classConstructor" , which forces the constructor to be called with new: What is the reason ES6 class constructors can't be called as normal functions?

Class methods are non-enumerable. Can be done with Object. Classes always use strict. Can be done with an explicit use strict for every function, which is admittedly tedious. Here is a short example. Hope it's also helpful for you to understand JavaScript Prototype Chain. Every function has a prototype property, initially holding an empty object. Objects created with new are linked to the prototype property of their constructor.

If a function is never used as a constructor, its prototype property will go unused. If you don't need a constructor, use Object. Revision 5 removed some useful info, including info on Object. See revision 4.

Palec what should I add back? IMO at least the link to Object. And I liked your examples of how prototypes work with constructors and Object. Javascript doesn't have inheritance in the usual sense, but it has the prototype chain. It's also useful for inheritance, because the prototype chain can consist of many other objects. Georg, please clarify for a noob - "there is no difference between classes and instances in javascript.

How does this work? Example: Attach property to object. Try it. Example: prototype. The prototype property is special type of enumerable object which cannot be iterate using for.. Example: Object's prototype. Property Description constructor Returns a function that created instance. It returns prototype object of a function to which it links to. Method Description hasOwnProperty Returns a boolean indicating whether an object contains the specified property as a direct property of that object and not inherited through the prototype chain.

Example: Changing Prototype. Want to check how much you know JavaScript? Start JavaScript Test. Share Tweet Share Whatsapp.

This is invisible property of an object. Returns a boolean indicating whether an object contains the specified property as a direct property of that object and not inherited through the prototype chain.