Conditional middleware execution in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is middleware in Express?

Middleware is a function that runs during the request-response cycle. It can modify the request or response, or end the request.

beginner

How can you run middleware only for certain routes in Express?

You can attach middleware only to specific routes by passing it as an argument before the route handler, so it runs conditionally.

intermediate

How do you conditionally execute middleware based on a request property?

Inside the middleware, check the request property (like req.path or req.method). Call next() only if the condition is met; otherwise, skip or respond.

beginner

What does calling next() do in Express middleware?

Calling next() passes control to the next middleware or route handler in the stack.

intermediate

Give an example of conditional middleware execution in Express.

Example: Use middleware only if req.query.admin is 'true'. Inside middleware, check if req.query.admin === 'true', then call next(); else respond with 403.

What is the purpose of middleware in Express?

A To store data permanently

B To style HTML pages

C To create database connections

D To handle requests and responses during the request cycle

Middleware functions run during the request-response cycle to process requests or responses.

How do you run middleware only for a specific route in Express?

A Attach middleware as an argument before the route handler

B Write middleware inside the route handler

C Use middleware globally for all routes

D Middleware cannot be route-specific

Passing middleware before the route handler runs it only for that route.

What happens if you don't call next() inside middleware?

A The request will hang and not proceed

B The next middleware runs automatically

C The server restarts

D The response is sent automatically

Without calling next(), Express does not continue to the next middleware or route.

Which property can you check to conditionally run middleware?

A console.log

B req.method

C res.statusCode

D process.env

req.method holds the HTTP method and can be used to conditionally run middleware.

How can you skip middleware execution for some requests?

A Middleware always runs for all requests

B Remove middleware from the app

C Check condition inside middleware and call next() only if true

D Use a different server

Inside middleware, you can decide to call next() or respond early based on conditions.

Explain how to run middleware conditionally in Express based on the request path.

Think about how to decide inside middleware whether to continue or stop.

Describe the role of next() in Express middleware and what happens if it is not called.

Consider the flow of request handling in Express.