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Understanding Express Middleware

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What is Express Middleware?

2:05 with Joel Kraft

Learn what middleware does in Express and what it looks like in code.

Prerequisite Course

For best results, we recommend first taking this prerequisite course...

HTTP Request Methods (GET, POST, etc)

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    [MUSIC] 0:00
    Express is described as a routing and middleware web framework. 0:05
    When you work with Express you're almost always working with middleware. 0:09
    So what is middleware? 0:13
    Consider when someone asks you a question. 0:15
    You hear the question, think about it for a moment and then answer the question. 0:18
    That middle step, the thinking, is analogous to what middleware does. 0:22
    An Express application receives requests and sends responses. 0:27
    If we open up an Express app to see what happens between a request and 0:33
    a response, it sort of works like a conveyor belt. 0:37
    The request comes in at the beginning and the response leaves at the end. 0:40
    All along the conveyor belt, middleware acts on the requests, 0:45
    packaging up a response to send back to the user. 0:48
    There can be as many pieces of middleware as you want to have 0:51
    before sending a response. 0:55
    In terms of code, middleware is a function with three parameters, 0:57
    request, response and next. 1:01
    Request and response are objects that can be read and modified in the function. 1:04
    Middleware can use the information in request 1:09
    to work out how to handle a response. 1:12
    Next is a function that must be called when the work is done. 1:15
    This triggers the next middleware function to execute. 1:19
    Most programmers shorten their request and response parameter names, to req and res. 1:22
    Just as clear a name, but with a lot less typing. 1:28
    To run middleware in response to requests, pass it to app.use. 1:31
    This will run the middleware function for every request. 1:36
    To only run it for 1:39
    a specific route, pass a route argument in before the middleware function. 1:40
    You can also limit the middleware to run only when 1:45
    a get request is made by using get instead of use. 1:48
    Get is the most common type of request made by browsers. 1:52
    If you want to look at other request types, check the teachers notes. 1:56
    In the next video, let's write some middleware to get a feel for how it works. 2:00

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