Express.js
/Beginner
What is Middleware?
Definition
Middleware functions are functions that have access to the request object (`req`), the response object (`res`), and the `next` function in the application's request-response cycle. Middleware can execute any code, make changes to the req/res objects, end the response cycle, or call `next()` to pass control to the next middleware in line.
Explain Like I'm New
Middleware functions are functions that have access to the request object (`req`), the response object (`res`), and the `next` function in the application's request-response cycle.
Capabilities of Middleware
| Action | Example Scenario |
|---|---|
| Execute code | Logging the time every request comes in. |
| Modify objects | Parsing a JWT and attaching the `user` to `req.user`. |
| End cycle | Sending a 401 Unauthorized response if a user is not logged in. |
| Pass control | Calling `next()` so the actual route handler can do its job. |
Interview Questions
basic
- What is the primary purpose of What is Middleware? in Express.js?
- How do you initialize What is Middleware??
intermediate
- How does What is Middleware? integrate with other middleware components?
- Can you explain a common use case for What is Middleware??
advanced
- What are the performance implications of What is Middleware? in a high-traffic production application?
- How would you debug issues related to What is Middleware??
trick
- Is it possible to achieve the same result as What is Middleware? without using Express?