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Let's write some Middleware right
into our app to see how it works.

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I'll just make some space
above our routes on line 12.

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You'll often pass Middleware is
an anonymous function into the app use

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method.

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We need the request,
response, and next arguments.

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Let's just do a log
statement to the console.

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Then we can call next.

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We'll look more closely at
the next function sin, but for

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now, it is enough to know that we're
passing control forward through the app.

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You can think of next as the next
step on the conveyor belt.

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This middleware runs every time
a request comes into the app.

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Switch over to the browser,
and refresh the page.

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Nothing changes on the page,
but if we look in the console,

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you will see our message is logged.

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Now we'll add another middleware function.

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Just copy this below and
change the message.

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When I save and refresh the browser,
the terminal shows

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two messages in the order
which they appear in the code.

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It's important to know that you
can depend on the sequence.

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Earlier middleware functions,
runs before the later ones.

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You can parse several functions into
the same app.use method code and

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they will run in the order
they're parsed in.

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Let's add another function
to the first app.use.

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We'll copy the function,
put a comma, and then paste.

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This will log between one and two.

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So let's say one and a half.

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Save, refresh, and check the console.

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Each middleware function printed
in the order we expected.

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So we've just seen two ways to include
middleware functions in the app.

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You can call the app.use as
many times as you'd like,

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passing in a function to each call.

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You can also parse in many functions
as you want into a single app.use call.

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Express will always run functions
in the order they appear.

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Let's trigger the middle word based on
the URL or route that a user visits.

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We can do this by placing the URL as
the first argument to the app.use method.

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Let's wrap the first app.use,
two slash one.

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Refreshing the browser,

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we see that the terminal only
showed the string two logged.

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That's because this middleware runs
with every request no matter what.

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If I clear the console and go to /one
in the browser, You see an error.

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As we've seen, that just means we haven't
set up a route handler for this URL.

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But what we're really interested in,
is the console at this point.

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Switching back to the console,
we have One, One and a half, and Two.

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You can also pass information
between functions.

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I'll delete these routes, And

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these second middleware arguments for now.

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Let's use the request object to pass the
dates from the first middleware function

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to the next middleware function.

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In the first function, we'll need to
create a property called message.

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Let's assign the string
'This message made it!'.

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Message isn't a special name.

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I could call it anything or
add more properties if I wanted.

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The idea here Is that we'll
use the property to pass

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as message from one middleware
function to the next.

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In the next middleware function
I can read the message back

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from the property and log it out.

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Let me check to see if it works.

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And it does.

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We just modified the request object for
the first time.

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That's a lot of what gives
middleware its power.

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Allowing us to gather and compute data and
send it back to the client.

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BodyParser for example, creates a body
property on the request object,

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and then we can use that
to respond to the client.

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Let's take a closer look at the next
function in the next video.