What is Express Middleware?2:05 with Joel Kraft
Learn what middleware does in express, and what it looks like in code.
[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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