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Middleware

Middleware is a way to wrap a handler with additional functionality. It is a common pattern in web applications that allows us to write DRY code. For example, we can write a middleware that logs every request to the server. We can then wrap our handler with this middleware and every request will be logged without us having to write the logging code in every handler.

Middleware in Express

Middleware in Express has the following function signature:

type Middleware = (req: Request, res: Response, next: NextFunction) => void;

req: The request object.
res: The response object.
next: A function that, when called, will pass control to the next middleware in the chain.

You can register middleware on an application level using the express app's .use method.

app.use(middlewareLogging);

You can also apply it to specific routes by passing it as an argument to the route's handler.

app.get("/users", middlewareLogging, handlerGetUsers);

In fact, you can use as many middleware functions as you like:

app.get('/users', middlewareLogging, middlewareAuth, ...,  handlerGetUsers);

Assignment

There's been an increase in non-OK responses in the Chirpy API. The developer team's KPIs are suffering, and they need to know what's going on. They've asked you to write a middleware that logs every request that returns a non-OK status code.

Create a middleware function called middlewareLogResponses that matches the function signature for middleware in express.
Within the function use the Response object's .on method. This method allows you to listen for events on the response object. Listen for the finish event.

res.on("finish", () => {
  //...
});

Express's Response object extends Node's http.ServerResponse, which is a writable stream. This means you can listen for events on it.

Grab the status code from the response object's statusCode property.
If the status code is non-OK, log the following to the console,

[NON-OK] <http_method> <url> - Status: <status_code>

Replace <http_method>, <url>, and <status_code> with the actual values. You can get these with method and url properties on the Request object.

Call next() at the end of the middleware.
Use the middleware at the application level. Since it's subscribing to all finish events, it doesn't matter where you set it.
Run the server and tee the output (copies the stdout) to a new file called server.log (and .gitignore the log).

npm run dev | tee server.log

Run and submit the CLI tests from the project directory in another terminal.