Original subtitles

In the ;ast couple of lectures,

I hope you started getting the fundamentals

of Express development.

And so now is the perfect time to dive a bit deeper

into how Express works,

and for that we need to talk about middleware

and the request-response cycle.

So really the essence of Express development

is to understand

and then use request-response cycle.

So I wanna make that idea really clear

in a visual way in this video.

So to start the request-response cycle,

or Express app receives a request when someone hits a server

for which it will then create a request

and response object.

That data will then be used and processed

in order to generate and send back a meaningful response.

Now, in order to process that data,

in Express we use something called middleware,

which can manipulate the request or the response object.

Or really execute any other code that we like.

So middleware doesn't always have to be just

about the request or the response object,

but it usually is mostly about the request.

And actually, we already used middleware before.

We used Express dot JSON to get access to the request body

on the request object.

Remember that?

Now it's called middleware remember

because it's a function that is executed between,

so in the middle of receiving the request

and sending the response.

And actually, we can say that in Express,

everything is middleware.

Even our route definitions.

So again, even when we defined our routes,

we can think of the route handler functions that we wrote

as middleware functions.

They are simply middleware functions

that are only executed for certain routes.

Okay?

Now some examples of middleware are Express dot JSON,

which is also called body-parser,

and that we already used before.

Or some logging functionality,

or setting some specific http headers.

The possibilities are really endless with middleware.

All right, and now in more technical terms,

we say that all the middleware together

that we use in our app,

is called the middleware Stack.

What's very important to keep in mind here,

is that the order of middleware in the stack,

is actually defined by the order they

are defined in the code.

So a middleware that appears first in the code,

is executed before one that appears later.

And so the order of the code matters a lot in Express.

Okay?

Now, you can think of the whole process like this,

our request and response object

that were created in the beginning

go through each middleware where they are processed,

or where just some other code is executed.

Then at the end of each middleware function,

a next function is called,

which is a function that we have access to

in each middleware function.

Just like the request and response objects.

And we will of course see this in code in the next video.

So when we call the next function,

the next middleware in the stack will be executed

with the exact same request and response object.

And that happens with all the middlewares

until we reach the last one.

And so just like this,

the initial request and response object

go through each middleware step by step.

And you can think of this whole process

as kind of a pipeline where our data go through,

so just like its been piped from request to final response.

All right?

Now about that last middleware function,

it's usually a route handler just like we coded before.

So in this handler we do actually not call the next function

to move to the next middleware.

Instead, we finally send the response data

back to the client.

And like this,

we then finish the so-called request-response cycle.

So the request-response cycle is really everything

that we talked about here together.

It starts with the incoming request,

then executing all the middleware in the middleware stack

step by step,

and finally sending the response to finish the cycle.

So you see, it's really not complicated.

It's actually just a linear process.

But I wish someone would have shown me something like this

when I was learning how to build Express apps.

I mean I could build them,

but in the beginning I didn't really understand

this entire request-response cycle like this.

It was very confusing.

So that's why I'm showing you this,

because I believe it will be much, much, easier for you

in order to move onto the course now,

after understanding exactly how Express apps work.