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In the docs for the NodeJS express module, the example code has app.use(...).
What is the use function and where is it defined?
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app.use(path, callback) Parameters: path: It is the path for which the middleware function is being called. It can be a string representing a path or path pattern or regular expression pattern to match the paths. callback: It is a middleware function or a series/array of middleware functions.
The app. use() method mounts or puts the specified middleware functions at the specified path. This middleware function will be executed only when the base of the requested path matches the defined path.
app. get is called when the HTTP method is set to GET , whereas app. use is called regardless of the HTTP method, and therefore defines a layer which is on top of all the other RESTful types which the express packages gives you access to.
json() is a built-in middleware function in Express. This method is used to parse the incoming requests with JSON payloads and is based upon the bodyparser. This method returns the middleware that only parses JSON and only looks at the requests where the content-type header matches the type option.
Node.js is an open-source and cross-platform runtime used when executing JavaScript code on the server-side. One of the popular Node.js server frameworks is Express.
The `app.use ()` Function in Express. Nov 16, 2020. Express apps have a use () function. This function adds a new middleware to the app. For example, suppose you want to print the HTTP method (get, post, etc.) and the URL of every request.
After initializing your App like let app = express();, you can find below some examples: app.use(...) As you correctly pointed, it is useful for "middlewares", it will apply to all the GETs, POSTs, etc. you indicate afterwords.
Each app.use (middleware) is called every time a request is sent to the server. app.use () acts as a middleware in express apps. Unlike app.get () and app.post () or so, you actually can use app.use () without specifying the request URL.
The app object is instantiated on creation of the Express server. It has a middleware stack that can be customized in app.configure()(this is now deprecated in version 4.x).
To setup your middleware, you can invoke app.use(<specific_middleware_layer_here>) for every middleware layer that you want to add (it can be generic to all paths, or triggered only on specific path(s) your server handles), and it will add onto your Express middleware stack. Middleware layers can be added one by one in multiple invocations of use, or even all at once in series with one invocation. See use documentation for more details.
To give an example for conceptual understanding of Express Middleware, here is what my app middleware stack (app.stack) looks like when logging my app object to the console as JSON:
stack: [ { route: '', handle: [Function] }, { route: '', handle: [Function: static] }, { route: '', handle: [Function: bodyParser] }, { route: '', handle: [Function: cookieParser] }, { route: '', handle: [Function: session] }, { route: '', handle: [Function: methodOverride] }, { route: '', handle: [Function] }, { route: '', handle: [Function] } ] As you might be able to deduce, I called app.use(express.bodyParser()), app.use(express.cookieParser()), etc, which added these express middleware 'layers' to the middleware stack. Notice that the routes are blank, meaning that when I added those middleware layers I specified that they be triggered on any route. If I added a custom middleware layer that only triggered on the path /user/:id that would be reflected as a string in the route field of that middleware layer object in the stack printout above.
Each layer is essentially adding a function that specifically handles something to your flow through the middleware.
E.g. by adding bodyParser, you're ensuring your server handles incoming requests through the express middleware. So, now parsing the body of incoming requests is part of the procedure that your middleware takes when handling incoming requests -- all because you called app.use(bodyParser).
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Each app.use(middleware) is called every time a request is sent to the server.
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