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Express Basics

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What is Express Middleware?

2:22 with Andrew Chalkley and Joel Kraft

You've already been using it, now learn what Express middleware is!

Documentation

Basic form of Express middleware

(req, res, next) => {
  // do something
  next();
}

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    [MUSIC] 0:00
    We already used some Express Middleware in our project. 0:04
    We use the body parser to post incoming form data and 0:08
    the cookie parser to read cookie information. 0:12
    But that's not all you can do with Middleware. 0:16
    In fact, Middleware is so integral to Express, 0:18
    just about everything you write in Express is Middleware. 0:21
    So what is it? 0:24
    Consider when someone asks you a question, you hear the question, think about it for 0:25
    a moment, and then answer the question. 0:30
    If we relate this to the request response cycle, that middle step, 0:33
    the thinking, is analogous to what Middleware does. 0:37
    An Express application receives requests and sends responses. 0:42
    You can sort of think of Express' request response cycle like a conveyor belt. 0:46
    The result comes in at the beginning and the response leaves at the end. 0:51
    All along the conveyor belt Middleware acts upon the requests, 0:55
    packaging up a response to send back to the end user. 0:59
    There can be as many pieces of Middleware as you want to have before sending 1:03
    a response. 1:07
    The basic structure of Middleware code is very simple. 1:09
    It's a function with three parameters, request, response and next. 1:13
    Inside the function, Middleware can read and modify the request and 1:19
    response objects. 1:22
    For example, the cookie parser modified the response object, 1:23
    placing the cookie's contents onto it. 1:27
    Next is a function that must be called, when the work is done. 1:31
    This triggers the Middleware function after the current one to execute. 1:34
    To run Middleware in response to requests, pass it into app.use. 1:39
    This will run the Middleware function for every request. 1:45
    To only run it for 1:48
    a specific route, pass the router argument in before the Middleware function. 1:49
    You can also limit the Middleware to only use get requests. 1:56
    This is done by using get instead of use. 2:01
    Hopefully this is looking familiar. 2:04
    This is very similar code to what we've been running to handle our routes. 2:07
    The only difference is that we haven't been declaring or using next yet, but 2:11
    we'll get to that soon. 2:15
    In the next video, let's write some Middleware to get a better feel for 2:18
    how it works. 2:21

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