Q1. What is middleware in Express.js?
Middleware functions are functions that have access to the request (req), response (res), and the next middleware function (next) in the application's request-response cycle.
They can execute code, modify req/res, end the request, or call
They are the building blocks of Express applications.
They can execute code, modify req/res, end the request, or call
next().They are the building blocks of Express applications.
Q2. What are the types of middleware in Express?
There are several types:
• Application-level middleware (bound to app using
• Router-level middleware
• Built-in middleware (
• Error-handling middleware (takes four parameters)
• Third-party middleware (like body-parser, cors)
• Application-level middleware (bound to app using
app.use())• Router-level middleware
• Built-in middleware (
express.json(), express.static())• Error-handling middleware (takes four parameters)
• Third-party middleware (like body-parser, cors)
Q3. How does the middleware execution flow work?
Middleware executes in the order they are defined.
Each middleware must either send a response or call
If a middleware doesn't call
The flow is like a pipeline.
Each middleware must either send a response or call
next() to pass control to the next middleware.If a middleware doesn't call
next() and doesn't send a response, the request hangs.The flow is like a pipeline.
Q4. What is the purpose of the next() function?
If you pass an argument to
Without calling
next() passes control to the next middleware function.If you pass an argument to
next(), like next(err), Express treats it as an error and skips to error-handling middleware.Without calling
next(), the request-response cycle stops at that middleware.Q5. Can middleware be applied to specific routes?
Yes, you can pass middleware as arguments to route methods.
Example:
Here
You can also chain multiple middleware functions.
Example:
app.get('/profile', authenticate, (req, res) => {...});Here
authenticate is a middleware that runs only for this specific route.You can also chain multiple middleware functions.