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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.
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.
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.
Why use path.join over string concatenation
Path.join uses the correct OS separator, collapses duplicate slashes, normalizes . and .. segments.
Minimal HTTP server with the http module
CreateServer with a request listener, set status and Content-Type, end the response, call listen on 3000.
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.
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.
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.
fs.watch vs fs.watchFile
Watch uses OS event notifications, efficient but inconsistent across platforms, watchFile polls stat at an interval, reliable but slower.
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.
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.
http.Agent and connection pooling
The agent pools and keeps sockets alive, avoiding repeated TCP and TLS handshakes, controlled by keepAlive and maxSockets.
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.
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.
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.
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.
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.
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.
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.
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.