⚙️ Backend Dev

Application-level vs router-level middleware

App.use() binds to the app and runs everywhere; router.use() binds to a Router and runs only for that router's routes.

Write a request-logging middleware in Express

Read req.method, log with a timestamp, then call next() to continue.

Parse JSON and URL-encoded bodies in Express

Express.json() for JSON, express.urlencoded() for form data, both registered via app.use().

What is Express middleware

Middleware are functions in a chain with req, res, next, they inspect or modify request and response, then call next to continue or send a response to end.

Centralized error-handling middleware

Error middleware takes four args err, req, res, next, is defined last, and runs when next(err) is called or sync errors throw.

Custom API key auth middleware

Read the header from req, on missing or invalid send res.status(401) and return, on valid call next, mount before protected routes.

Modularizing routes with express.Router

Create a Router instance in users.js, attach routes to it, export it, then mount it under a base path with app.use in the main file.

Express route order and matching

Express checks routes in definition order, /items/new matches the literal route first, if /:id came first new would be captured as an id.

res.send vs res.json vs res.end

Send is flexible and sets content type by type, json serializes and sets JSON content type, end is the raw http terminator with no body helpers.

Parsing JSON bodies with express.json

Register express.json via app.use so it reads the request stream and populates req.body before handlers run, place it before routes.

Query params vs route params in Express

Query string values via req.query.q, named path segments via req.params.id, query is optional, route params are part of the matched pattern.

Serving static files in Express

App.use with express.static pointing at the public directory, files served relative to that root, often combined with an absolute path.

Minimal Express Hello World server

Import express, create an app, define app.get on the root sending a response, call app.listen on a port.

http.Agent and connection pooling

The agent pools and keeps sockets alive, avoiding repeated TCP and TLS handshakes, controlled by keepAlive and maxSockets.

Backpressure in Node.js streams

Backpressure pauses the readable when the writable buffer fills, pipe and pipeline manage it automatically, pipeline also propagates errors and cleans up.

Scaling across cores with cluster and os

Os.cpus gives core count, the primary forks one worker per core, all workers share the listening port, the OS load-balances connections.

fs.watch vs fs.watchFile

Watch uses OS event notifications, efficient but inconsistent across platforms, watchFile polls stat at an interval, reliable but slower.

path.resolve vs path.join

Join concatenates and normalizes relative segments, resolve builds an absolute path right to left from cwd and resets on any absolute segment.

Reading a POST body from the request stream

Body arrives in chunks via data events, accumulate them, on the end event concatenate and JSON.parse inside try/catch.