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Express is referred to as a routing and
middle way framework.

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Routing lets our app do different things
and display different information,

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based on which you are at the browser
requests, and how it requests it.

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So sending a GET request to the slash
login route leads to the login form.

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While a PUT request to slash login,
tells the app to authenticate the user

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by comparing the form data
against records in the database.

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Middleware is software that sits in the
middle of other parts of your application.

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In other words, it's programming that
runs after a request is received, but

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before a response is sent back.

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We've already seen a few
examples of middleware.

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For instance, most express apps ours
included use the body parser module

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to convert incoming requests
into a format that's easy for

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a JavaScript program to use.

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The Express app gets a request.

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Body parser takes the body of the request,
makes it readable to our program

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then hands it off to the next
part of the Express application.

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It sits in the middle
of that request cycle.

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Another example of middleware in our
application is the Express session module

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we used to track sessions for
logged in users.

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This is called application
level middleware,

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because it makes sessions
available anywhere in our app.

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You'll often use more than
one piece of middleware.

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In fact, you can chain together
multiple middleware functions

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that run one after the other.

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In Express, middleware is a function
that has access to the request and

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response objects.

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The entire process from
request to response in Express

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is called the request response cycle.

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A typical piece of middleware is
a function that accepts three arguments,

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the request object, the response object
and a third parameter named next.

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Next is a function, and it represents
the next piece of middleware,

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the next function in
the request response cycle.

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In our app, we've added a bit of
middleware to make sure that the user ID

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property of the session is
available to the application.

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This middleware function assigns a user
ID to the current user property.

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When it's done,

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it calls next, telling express to move
on to the next piece of middleware.

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When writing custom middleware,
we must always include a call to next.

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Let's take a closer look at the middleware
we're already using in our app.

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If you look in the app.js file,
you can see multiple app.u statements.

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These add middleware to the application.

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Some are third party middleware,
like session and body parser, and

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some are built into Express like
Express.static which lets a sort of

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static files within a specific directory,
like images and CSS files.

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Also, towards the bottom of the file,
there's error handling

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middleware that defines middleware
function in line as an anonymous function.

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It accepts an error object and
serves that to the user.

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You can see that the error handling
middleware takes four parameters,

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the usual request, response,
and next parameters but

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the first argument is an error object.

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There's also router middleware.

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That's a function you add
to particular routes.

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You can find links to more
information about Express middleware

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in the teachers notes for this video.

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Next, we'll write custom middleware for
our user authentication system.

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Specifically, we'll create router
middleware to redirect visitors

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based on their logon status.