Understanding Express Middleware

Express is referred to as a routing and middle way framework. 0:00 Routing lets our app do different things and display different information, 0:03 based on which you are at the browser requests, and how it requests it. 0:07 So sending a GET request to the slash login route leads to the login form. 0:12 While a PUT request to slash login, tells the app to authenticate the user 0:17 by comparing the form data against records in the database. 0:21 Middleware is software that sits in the middle of other parts of your application. 0:25 In other words, it's programming that runs after a request is received, but 0:30 before a response is sent back. 0:34 We've already seen a few examples of middleware. 0:36 For instance, most express apps ours included use the body parser module 0:39 to convert incoming requests into a format that's easy for 0:44 a JavaScript program to use. 0:48 The Express app gets a request. 0:50 Body parser takes the body of the request, makes it readable to our program 0:53 then hands it off to the next part of the Express application. 0:57 It sits in the middle of that request cycle. 1:01 Another example of middleware in our application is the Express session module 1:04 we used to track sessions for logged in users. 1:08 This is called application level middleware, 1:11 because it makes sessions available anywhere in our app. 1:14 You'll often use more than one piece of middleware. 1:17 In fact, you can chain together multiple middleware functions 1:20 that run one after the other. 1:23 In Express, middleware is a function that has access to the request and 1:26 response objects. 1:30 The entire process from request to response in Express 1:32 is called the request response cycle. 1:35 A typical piece of middleware is a function that accepts three arguments, 1:38 the request object, the response object and a third parameter named next. 1:42 Next is a function, and it represents the next piece of middleware, 1:48 the next function in the request response cycle. 1:51 In our app, we've added a bit of middleware to make sure that the user ID 1:54 property of the session is available to the application. 1:58 This middleware function assigns a user ID to the current user property. 2:02 When it's done, 2:06 it calls next, telling express to move on to the next piece of middleware. 2:06 When writing custom middleware, we must always include a call to next. 2:11 Let's take a closer look at the middleware we're already using in our app. 2:18 If you look in the app.js file, you can see multiple app.u statements. 2:22 These add middleware to the application. 2:28 Some are third party middleware, like session and body parser, and 2:30 some are built into Express like Express.static which lets a sort of 2:36 static files within a specific directory, like images and CSS files. 2:41 Also, towards the bottom of the file, there's error handling 2:48 middleware that defines middleware function in line as an anonymous function. 2:51 It accepts an error object and serves that to the user. 2:56 You can see that the error handling middleware takes four parameters, 3:00 the usual request, response, and next parameters but 3:03 the first argument is an error object. 3:08 There's also router middleware. 3:12 That's a function you add to particular routes. 3:14 You can find links to more information about Express middleware 3:16 in the teachers notes for this video. 3:19 Next, we'll write custom middleware for our user authentication system. 3:22 Specifically, we'll create router middleware to redirect visitors 3:26 based on their logon status. 3:29