Javascript how does prototype work

In this part I'm going to take all the steps JavaScript takes when using the new keyword, but I'm not going to use the new keyword and prototype. So when we do new Person 'George' , Person function serves as a constructor, these are what JavaScript does, one by one:. This is not the way that JavaScript actually attaches the properties that are defined in the prototype.

The actual way is related to the prototype chain concept. Then the constructor can do whatever it wants, because this inside of that constructor is the object that was just created. Basically, when you use the new keyword on a function, you are calling on that and that function serves as a constructor, so when you say:. 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. How does JavaScript. Ask Question. Asked 12 years, 8 months ago. Active 1 year, 1 month ago. Viewed k times. 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. 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? Objects The object datatypes can be further divided into two types Function type objects Non Function type objects The Function type objects are the ones that return the string 'function' with typeof operator.

The object referenced by UserDefinedFunction. Hope this helps. Arup Hore Arup Hore 1, 14 14 silver badges 9 9 bronze badges. We have modified the Root and added a function branch to it. We can also add primitives or objects to our Prototype. Let's add a child-tree to our Tree.

In JS everything is not an object, everything can act like an object. Javascript supports inheritance via delegation based on prototypes.

Each Function has a prototype property, which refers to another object. Thalaivar Thalaivar It may help to categorise prototype chains into two categories.

Here is a visual presentation of the two prototype chains involved, represented as columns: To summarise: The prototype property gives no information of the subject's prototype chain, but of objects created by the subject. You can jump back and forth between the two prototype chains: Person. So two objects are created at the same time: The function Person itself The object that will act as prototype when the function is called as a constructor Both are objects, but they have different roles: the function object constructs , while the other object represents the prototype of any object that function will construct.

Bad Bad 4, 3 3 gold badges 29 29 silver badges 48 48 bronze badges. B M B M 3, 3 3 gold badges 24 24 silver badges 41 41 bronze badges. Tom Tom 3 3 silver badges 4 4 bronze badges. There's two distinct but related entities here that need explaining: The. The [[Prototype]] [1] property of all objects [2]. These are two different things. The [[Prototype]] property: This is a property that exists on all [2] objects.

Palec Dimitris Fasarakis Hilliard Dimitris Fasarakis Hilliard k 27 27 gold badges silver badges bronze badges. Properties in the product are handled in the following way: When a property is accessed to just read it's value, its looked up in the scope chain. The search for the variable starts from the product upwards to it's prototype.

If such a variable is found in the search, the search is stopped right there, and the value is returned. If such a variable cannot be found in the scope chain, undefined is returned. When a property is written altered , then the property is always written on the product object. Finally, a new Rectangle object called rect is created. It inherits from Rectangle. There is something misleading in JavaScript regarding prototypes. The prototype of an object is not the same thing as the prototype property of the object.

The former is used when looking up non-existent properties in the prototype chain. The latter is used for objects created using new , it will be the prototype of the newly created object. Understanding this difference allows you to fully understand the prototype property in JavaScript. 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. Returns a boolean indication whether the specified object is in the prototype chain of the object this method is called upon.