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‫As the last part in this section,

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‫let's talk about how NODE.JS modules actually work

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‫behind the scenes.

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‫First of all in the NODE.JS module system

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‫each JavaScript file is treated as a separate module.

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‫And we already played around a bit with modules

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‫in the intersection,

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‫'specially by using the require function

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‫and exporting data from modules.

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‫So that's the fundamentals of Common JS modules

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‫which is the module system that node uses.

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‫Because it works especially well on the server.

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‫Now, there's also the native ECMA script

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‫or ES module system that

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‫you might already be familiar with,

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‫'specially if you took my JavaScript course,

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‫or if you have been writing JavaScript

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‫on the front end lately.

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‫This ES module system that I was talking about was developed

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‫to work in the browser using the import export syntax.

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‫So, I just wanted to make it clear that these are

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‫two different module systems that you will see

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‫in different situations,

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‫so don't be confused.

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‫There have also been attempts to implement

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‫native ES modules in NODE.JS,

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‫'specially using file extensions

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‫like .mjs.

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‫But so far, it's not really become

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‫a popular thing to use.

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‫So unless you're watching this course

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‫in the far future,

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‫you will probably always be using

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‫the Common JS system with node.

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‫Now, you might be wondering why in NODE.JS

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‫each and every module actually gets access

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‫to the require function in order

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‫to import modules in the first place.

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‫Right?

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‫I mean, it's not a standard JavaScript function.

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‫So where does it come from?

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‫And how exactly does it work behind the scenes?

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‫Well, let's find out.

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‫So the question here is,

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‫what happens each time that we require

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‫a module by calling the require function

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‫with a module name, is the argument.

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‫As a very broad overview,

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‫the following steps are executed behind the scenes.

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‫First, the path to the required module

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‫is resolved and the file is loaded.

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‫Then a process called wrapping happens

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‫after that,

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‫the module code is executed

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‫and the module exports are returned.

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‫And finally the entire module gets cached.

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‫So let's now look at each step in more detail.

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‫First off, how does node know which file to load

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‫when we require a module?

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‫Because remember, we can actually load

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‫three different kinds of modules.

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‫Node's core modules,

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‫our own modules or third party modules from npm

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‫like express which is the node framework that we're

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‫going to use throughout most of the course.

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‫So this process is known as resolving the file path.

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‫And here is how it works.

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‫When the require functions receives the module name

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‫as its input,

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‫it will first try to load a core module with that name.

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‫Like for example, the http module.

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‫It will automatically find a path to that module

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‫and then load it.

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‫Now if the path starts with a dot,

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‫or two dots,

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‫it means that it's a developer module.

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‫Because we indicate the relative path to our file.

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‫So node will try to load that file,

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‫but if there is no file with that name,

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‫then it will look for a folder instead

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‫and load index.js if it's in that folder.

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‫So in this example,

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‫it will try to load lib/controller.js.

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‫Remember that we can drop the dot js

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‫when requiring a module.

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‫In case there is no file at this path,

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‫it will then, instead, try to open

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‫index.js from the lib/controller folder instead.

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‫Make sense?

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‫And finally, if the required module

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‫is neither a core module,

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‫nor a developer module,

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‫node will assume it's a module from npm.

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‫And remember, on a module that we load from npm,

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‫we actually don't need to write the path when we require it.

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‫Just the module name,

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‫just the way it works on a core module.

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‫So these third party modules are stored

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‫in a node module folder,

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‫so node will step into that folder and try to find

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‫a module there and then load it.

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‫And of course, if the file can't be found anywhere,

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‫then an error is thrown and the execution

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‫of the app is stopped.

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‫Moving on, after the module is loaded,

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‫the module's code is wrapped into a special function

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‫which will give us access to a couple of special objects.

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‫So this step is where the magic happens.

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‫It is here where we get the answer to the question,

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‫where does the require function actually come from

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‫and why do we have access to it?

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‫It's because the nodejs run time,

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‫takes the code off our module

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‫and puts it inside the immediately invoked

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‫function expression, or IIFE that you can see here.

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‫So node does actually not directly

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‫the execute the code that I rewrite into a file,

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‫but instead, the wrapper function that will contain

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‫our code in it's body.

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‫It also passes the experts require module,

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‫file name and dir objects into it.

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‫So that is why in every module

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‫we automatically have access

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‫to stuff like the require function.

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‫So these are basically like global variables

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‫that are injected into each and every module.

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‫Now by doing this, node achieves two very important things.

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‫First, of course, is giving developers access

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‫to all these variables we just talked about,

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‫which is very helpful.

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‫Second, it keeps the top-level variables that we

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‫define in our modules private.

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‫So it's scoped only to the current module

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‫instead of leaking everything into the global object.

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‫So imagine for a second,

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‫that we declared a variable x in one of our modules

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‫and then included an npm module that also used

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‫a variable called x.

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‫Without this mechanism,

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‫our own variable would then get overridden

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‫and the entire app would just not work.

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‫So each module having its private scope

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‫is absolutely crucial and is achieved

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‫through this clever trick of wrapping our code

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‫into this special function.

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‫Let's now take a quick look at each object

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‫that our module gets.

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‫First the require function,

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‫which we already know about.

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‫Then, there is a module object,

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‫which is just a reference to the current module

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‫and which for us is most important

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‫when it comes to exporting data.

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‫The same applies to the export's object

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‫which we're gonna discuss in a moment.

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‫Finally, there are the convenience

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‫variables file name and dir name

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‫which simply contain the absolute path

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‫to the current module's file

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‫as well as the current directory name.

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‫And that is how the wrapping step of loading a module works.

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‫Next up, the code in the module,

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‫or to be more precise,

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‫in the module's wrapper function,

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‫actually gets executed by the NODE.JS runtime.

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‫So, nothing fancy here.

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‫Up until this point,

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‫the require function has been called with a module name

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‫as the argument the path

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‫to the module's file has been resolved into file loaded,

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‫all the code has been wrapped into the wrapper function

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‫which has then been executed.

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‫Now, it's time for the require function

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‫to actually return something.

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‫And what it returns is the exports of the required module.

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‫These exports are stored in the module.exportsobjects.

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‫It might look a little bit like this.

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‫So, again, in each module we can export variables,

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‫which will, in the end be returned by the require function.

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‫And we do that by assigning variables to module.exports

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‫or simply to exports

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‫where it is just a pointer to module.export.

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‫We will do this in practice in the next video,

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‫because I know this sounds kind of confusing

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‫when I just say it like this.

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‫Anyway, here's what you need to know

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‫about when to use module.export or just export.

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‫So if all you want to do is to export one single variable,

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‫like one class or one function,

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‫you usually use module.exports and set it equal

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‫to the variable that you want to export.

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‫Just like in this example.

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‫On the other hand,

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‫if you're looking to export multiple named variables

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‫like multiple functions, for example,

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‫then you should create these

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‫as properties of the export object.

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‫For example, if we were doing a calculator module

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‫you could create exports.add,

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‫exports.multiply,

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‫exports.divide and so on and so forth.

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‫Makes sense?

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‫Again, we're gonna do this in the next video

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‫and it will make more sense to you by then.

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‫This is effectively how we export and import data

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‫from one module into the other.

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‫Of course, this is also why we usually assign

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‫the result of a require function called to a new variable.

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‫So that we can save the returned exports.

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‫Now, just to finish this video real quick,

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‫the last step is that modules are actually cached

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‫after the first time they are loaded.

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‫What this means is that if you require the same module

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‫multiple times, you will always get the same result.

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‫And the code in the modules is actually only executed

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‫in the first call.

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‫In subsequent calls,

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‫the result is simply retrieved from cache.

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‫Perfect.

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‫So I hope that all the logic behind

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‫how to work with modules

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‫doesn't really look like magic anymore.

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‫It's really important to know how all of this works,

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‫in order to become a great,

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‫and above all,

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‫a more independent NODE.JS developer.