CodeParkMiddleware is a function (req, res, next) that runs as one step in the handling of a request. Express keeps an ordered list (a stack) of middleware and route handlers. Each request goes through the matching ones, in the order you registered them.
A middleware can do one of these things:
- Read or change
reqandres. - End the request by sending a response.
- Call
next()to pass control to the next matching function.
If it neither sends a response nor calls next(), the request hangs until the client times out.
next(err) with an argument skips all the remaining normal middleware. Express jumps straight to the error-handling middleware.
Typical middleware:
- Body parsing with
express.json() - Logging
- Authentication
- CORS
- Rate limiting
Registration order matters. For example, a body parser must come before the routes that read req.body, because otherwise req.body is not filled yet.
- Middleware has the signature
(req, res, next)and runs in registration order. - It can change req/res, end the response, or call
next(). - If it does neither, the request hangs.
next(err)jumps to the error handlers.- Order matters: for example, the body parser must come before the routes.