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[MUSIC]

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We already used some
Express Middleware in our project.

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We use the body parser to
post incoming form data and

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the cookie parser to
read cookie information.

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But that's not all you
can do with Middleware.

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In fact, Middleware is so
integral to Express,

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just about everything you write
in Express is Middleware.

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So what is it?

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Consider when someone asks you a question,
you hear the question, think about it for

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a moment, and then answer the question.

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If we relate this to the request
response cycle, that middle step,

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the thinking,
is analogous to what Middleware does.

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An Express application receives
requests and sends responses.

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You can sort of think of Express' request
response cycle like a conveyor belt.

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The result comes in at the beginning and
the response leaves at the end.

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All along the conveyor belt
Middleware acts upon the requests,

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packaging up a response to
send back to the end user.

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There can be as many pieces of Middleware
as you want to have before sending

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a response.

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The basic structure of
Middleware code is very simple.

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It's a function with three parameters,
request, response and next.

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Inside the function, Middleware can
read and modify the request and

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response objects.

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For example, the cookie parser
modified the response object,

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placing the cookie's contents onto it.

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Next is a function that must be called,
when the work is done.

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This triggers the Middleware function
after the current one to execute.

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To run Middleware in response to requests,
pass it into app.use.

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This will run the Middleware function for
every request.

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To only run it for

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a specific route, pass the router argument
in before the Middleware function.

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You can also limit the Middleware
to only use get requests.

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This is done by using get instead of use.

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Hopefully this is looking familiar.

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This is very similar code to what we've
been running to handle our routes.

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The only difference is that we haven't
been declaring or using next yet, but

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we'll get to that soon.

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In the next video, let's write some
Middleware to get a better feel for

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how it works.