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Awaiting four independent API calls in sequence instead of parallel can triple your endpoint latency. Here's the pattern, the math, and how to catch it in review.

You've read "Node.js is single-threaded and non-blocking." Fine. But that sentence raises more questions than it answers. If it's single-threaded, who's doing the file I/O while your JavaScript runs?

If you're still writing pages/api/... files in a new Next.js project, stop. Route Handlers are the App Router's replacement, and they're meaningfully better — typed, streaming-capable, and built on the same Web platform APIs that Cloudflare.

Strip Express and Koa down to their core: a 30-line compose() function that chains async middleware with before/after hooks, short-circuit support, and error propagation.

`forEach` doesn't return a Promise, so `await` before it resolves instantly. Every async callback fires and is forgotten — this is how batch processors ship broken.

Your endpoint is async but it still freezes the server. Here's the synchronous CPU work pattern that blocks Node's event loop and how senior devs catch it in review.

Node.js has two module systems that don't interoperate cleanly. Here's how CommonJS and ESM work, how Node resolves imports, and how circular dependencies behave in each.

EJS, Handlebars, Pug and Nunjucks all do the same three things: tokenize, parse, compile. Build a real one — loops, conditionals, partials, escaping by default — and see exactly where HTML escaping stops protecting you.

Open a modal 10 times without cleanup and you've got 10 stacked timers silently draining memory. Here's what this looks like in a PR and how senior devs catch it.