express middleware

Question 1

How do you mount a middleware with an express application ?

Answer 1

Middleware is the function passed to .use().

Question 2

Which objects does the express application pass to the middleware to manage the request/response lifecycle of a server?

Answer 2

The request object & the response object

Question 3

What is middleware and what can middleware do?

Answer 3

Middleware functions are functions that have access to the request object (req), the response object (res), and the next middleware function in the application’s request-response cycle. The next middleware function is commonly denoted by a variable named next.

Middleware functions can perform the following tasks:

Execute any code.
Make changes to the request and the response objects.
End the request-response cycle.
Call the next middleware function in the stack.
If the current middleware function does not end the request-response cycle, it must call next() to pass control to the next middleware function. Otherwise, the request will be left hanging.

Question 4

What does the express.json() middleware do and when would you need it?

Answer 4

This is a built-in middleware function in Express. It parses incoming requests with JSON payloads. Returns middleware that only parses JSON and only looks at requests where the Content-Type header matches the type option. A new body object containing the parsed data is populated on the request object after the middleware (i.e. req.body), or an empty object ({}) if there was no body to parse, the Content-Type was not matched, or an error occurred.