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The NodeJS Documentation states:

"The request object is an instance of IningMessage. The request object that's passed in to a handler implements the ReadableStream interface"

"So far we haven't touched on the response object at all, which is an instance of ServerResponse, which is a WritableStream."

JS has Prototypal Inheritance so when the documentation says it is an instance it means it adds to the prototype chain, but how it implements an Interface in JS?

And how is this all chained up and how it works. The Official NodeJS Documentation does not explain.

Source - Anatomy of an HTTP Transaction

Edit- My Question is not related to multiple Inheritance in JS. It is about how NodeJS modules implements interface which is natively not supported in JS. I beg your Pardon if there is any mistake in my question or lacking in my knowledge. Thanks!

The NodeJS Documentation states:

"The request object is an instance of IningMessage. The request object that's passed in to a handler implements the ReadableStream interface"

"So far we haven't touched on the response object at all, which is an instance of ServerResponse, which is a WritableStream."

JS has Prototypal Inheritance so when the documentation says it is an instance it means it adds to the prototype chain, but how it implements an Interface in JS?

And how is this all chained up and how it works. The Official NodeJS Documentation does not explain.

Source - Anatomy of an HTTP Transaction

Edit- My Question is not related to multiple Inheritance in JS. It is about how NodeJS modules implements interface which is natively not supported in JS. I beg your Pardon if there is any mistake in my question or lacking in my knowledge. Thanks!

• javascript
• node.js
• inheritance
• instance
• prototypal-inheritance

Share Improve this question Follow edited Feb 14, 2018 at 20:34 Mohammad Daud Ibrahim asked Feb 14, 2018 at 18:31 Mohammad Daud IbrahimMohammad Daud Ibrahim 14611 gold badge22 silver badges1313 bronze badges 4

• Possible duplicate of Multiple inheritance/prototypes in JavaScript – zero298 Commented Feb 14, 2018 at 18:36
• In traditional Object Oriented Oriented Programming like Java it implements class based Inheritance, Classes are extended and Interfaces are implemented. However JS implements Prototypal Inheritance. In JS inheriting means simply adding to the immediate prototype chain of an Object. I've not yet came across Implementing Interfaces in JS and that is why I've raised this question. Sorry, but your ment but was not helpful at all. – Mohammad Daud Ibrahim Commented Feb 14, 2018 at 20:28
• 2 @WittyHumour There's nothing in the language that enforces interfaces. When the documentation says "Implements the ReadableStream interface" it means that YOU as a human will go and read the documentation of ReadableStream and YOU as a programmer agree to write your code so that when other programmers use your object it will look like a ReadableStream object. It's just an informal agreement between humans. Not a language feature – slebetman Commented Feb 14, 2018 at 20:39
• The request object is an instance of IningMessage and it implements the ReadableStream interface. Can you please explain this in context with JavaScript. – Mohammad Daud Ibrahim Commented Feb 14, 2018 at 21:10

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1 Answer 1

Sorted by: Reset to default 7

Interface = Obligation.

To implement an interface = Provide members which are expected by these who call you.

What the docs say about this particular interface, the ReadableStream?

Well, here it says

All Readable streams implement the interface defined by the stream.Readable class

And what is the stream.Readable?

Well, here a plete "interface" is provided and it consists of multiple members, including read, pipe and others.

Technically, in JS there are multiple ways of "implementing" such obligation:

• an easy way is to set an object that already implements the interface in the prototype chain of an object that is supposed to implement the interface. This way all interface members are delegated (by means of the proto chain) to the other object

• a more direct way is just to provide all necessary members directly by the object that is supposed to implement the interface

• a hybrid approach where some members are delegated down the proto chain, some members are implemented directly and some members are delegated to other objects by direct invocation

Because of the nature of the language, the caller which expect the interface to be implemented will accept all these possibilities.

E.g. The docs says the interface contains a method

foo(n)
where n is a number 
returns a number

You have following options.

Option 1, a direct implementation

var o = {
    foo: function(n) {
       return 0;
    }
}

Option 2, delegation down the proto chain

// an object that already implements the interface
// i.e. has function foo(n)
var existing = ....; 

var o = Object.create(existing);

// o has function foo though its proto

Option 3, delegation by invocation

// an object that already implements the interface
// i.e. has function foo(n)
var existing = ....; 

var o = {
   foo: function(n) {
       return existing(n);
   }
}

All these options can possibly be expressed with construction functions, too.

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