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[MUSIC]

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Express is described as a routing and
middleware web framework.

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When you work with Express you're
almost always working with middleware.

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So what is middleware?

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Consider when someone asks you a question.

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You hear the question, think about it for
a moment and then answer the question.

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That middle step, the thinking,
is analogous to what middleware does.

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An Express application receives
requests and sends responses.

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If we open up an Express app to see
what happens between a request and

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a response,
it sort of works like a conveyor belt.

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

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All along the conveyor belt,
middleware acts on the requests,

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packaging up a response
to send back to the user.

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There can be as many pieces of
middleware as you want to have

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before sending a response.

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In terms of code, middleware is
a function with three parameters,

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request, response and next.

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Request and response are objects that can
be read and modified in the function.

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Middleware can use
the information in request

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to work out how to handle a response.

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

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This triggers the next
middleware function to execute.

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Most programmers shorten their request and
response parameter names, to req and res.

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Just as clear a name, but
with a lot less typing.

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To run middleware in response to requests,
pass it to 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 a route argument
in before the middleware function.

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You can also limit
the middleware to run only when

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a get request is made by
using get instead of use.

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Get is the most common type
of request made by browsers.

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If you want to look at other request
types, check the teachers notes.

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In the next video, let's write some
middleware to get a feel for how it works.