Node.js &amp; Express

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.

Counting lines in a 5GB log file efficiently

ReadFile loads all 5GB into RAM and may exceed buffer limits, instead stream with createReadStream plus readline and count line by line.

Minimal HTTP server with the http module

CreateServer with a request listener, set status and Content-Type, end the response, call listen on 3000.

Why use path.join over string concatenation

Path.join uses the correct OS separator, collapses duplicate slashes, normalizes . and .. segments.

fs.readFileSync vs fs.readFile

Sync blocks the event loop and returns directly, async takes a callback or promise, only sync is safe at startup.

Async iterators and for await...of for streaming

Async iterators yield values lazily over time; for await...of consumes them sequentially with backpressure, keeping memory bounded.

Promise.all vs Promise.allSettled

All rejects on the first failure; allSettled always fulfills with a status/value or reason per input. Use allSettled when partial success is acceptable.

advanced2 min read

Bounded concurrency for many async requests

Chunk the array and await Promise.all per chunk, or run a fixed worker pool pulling from a shared index; cap in-flight requests.

Comparing the three async error-handling styles

Callbacks pass err as first arg; Promises route errors to catch; async/await uses try/catch; an unhandled rejection can crash the Node process.

Handling async errors in Express middleware

Await inside try/catch and call next(err) on failure, or wrap the handler in an async error adapter; never let a rejected Promise go uncaught.

Running independent requests with Promise.all and race

Start all requests then await Promise.all to get all results or fail fast on first rejection; use Promise.race when only the fastest settled result matters.