ExpressJS Core Concepts of ExpressJS Syllabus

Core Concepts of ExpressJS syllabus — full chapter and topic list

Expand any chapter to see its topics and sub-topics. This is the whole examinable outline for Core Concepts of ExpressJS in ExpressJS, not a summary of it.

1. 1

   Middleware

   4 topics

   • What is Middleware?
   • Built-in Middleware
   • Third-party Middleware
   • Custom Middleware
2. 2

   Routing

   5 topics

   • Defining Routes
   • Route Methods
   • Route Parameters
   • Route Handlers
   • Express Router
3. 3

   Request and Response Objects

   4 topics

   • Understanding req Object
   • Understanding res Object
   • Handling Different Content Types
   • Sending Responses
4. 4

   Error Handling

   3 topics

   • Built-in Error Handling
   • Custom Error Handling Middleware
   • Debugging Tips

Core Concepts of ExpressJS flashcards for ExpressJS

18 of 51 cards from the Core Concepts of ExpressJS deck — real questions with worked answers.

1. What is middleware in Express.js?

   Middleware 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. They can execute code, modify req/res, end the cycle, or call next() to pass control onward.

2. What is the standard signature of an Express middleware function?

   function(req, res, next) { ... } — it receives the request object, the response object, and the next function. Error-handling middleware uses four arguments: (err, req, res, next).

3. What are the main types of middleware in Express?

   Application-level, router-level, error-handling, built-in, and third-party middleware.

4. What happens if a middleware neither ends the request nor calls next()?

   The request hangs — it is left pending because control is never passed on and no response is sent, so the client eventually times out.

5. How is middleware attached at the application level in Express?

   Using app.use() (runs for every request or a path) or a method like app.get()/app.post() bound to a specific route and HTTP verb.

6. Does middleware execution order matter in Express?

   Yes. Middleware runs in the exact order it is defined with app.use()/app.METHOD(). A middleware only affects requests reaching it, so ordering determines behavior (e.g., body parsers must run before route handlers that read req.body).

7. What is built-in middleware in Express?

   Middleware that ships with Express itself and requires no separate install, such as express.json(), express.urlencoded(), express.static(), and express.text()/express.raw().

8. What does express.json() do?

   It is built-in middleware that parses incoming requests with JSON payloads and populates req.body with the resulting JavaScript object.

9. What does express.urlencoded() do and what is the extended option?

   It parses URL-encoded form bodies (Content-Type: application/x-www-form-urlencoded) into req.body. extended: true uses the qs library allowing nested/rich objects; extended: false uses the querystring library allowing only simple key-value pairs.

10. What does express.static() do?

    It is built-in middleware that serves static files (HTML, images, CSS, JS) from a specified directory, e.g. app.use(express.static('public')).

11. What is third-party middleware and how is it added?

    Middleware written outside Express, installed via npm (e.g., npm install morgan) and then loaded with require() and mounted using app.use(). Examples: morgan, cors, helmet, cookie-parser, express-session.

12. Name three common third-party middleware packages and their purpose.

    morgan — HTTP request logging; cors — enabling Cross-Origin Resource Sharing; helmet — setting security-related HTTP headers. (Others: cookie-parser, express-session, multer.)

13. What is custom middleware and how do you write one?

    A user-defined function with signature (req, res, next) that performs some logic (logging, auth, validation) and then calls next(). Example: const logger = (req, res, next) => { console.log(req.method, req.url); next(); }; app.use(logger);

14. How do you restrict a custom middleware to only run for a specific path?

    Pass the path as the first argument to app.use(), e.g. app.use('/admin', authMiddleware) — the middleware then runs only for requests whose path starts with /admin.

15. What is the difference between built-in, third-party, and custom middleware?

    Built-in ships with Express (express.json, express.static); third-party is installed from npm (morgan, cors); custom is written by the developer for app-specific logic. All share the (req, res, next) signature.

16. How do you define a basic route in Express?

    Using app.METHOD(path, handler), e.g. app.get('/users', (req, res) => res.send('Users')). METHOD is the lowercase HTTP verb, path is the route pattern, and handler is the callback executed when the route matches.

17. What are the anatomy/components of an Express route?

    A route consists of an HTTP method (verb), a path (URL pattern), and one or more handler functions executed when the request matches both the method and the path.

18. What are Express route methods and which HTTP verbs do they correspond to?

    Methods on the app/router that map to HTTP verbs: app.get, app.post, app.put, app.delete, app.patch, app.head, app.options, etc. Each responds only to requests using that verb on the matching path.

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