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`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.

You have 200 images to resize. The resize API is fast, but it throttles at 10 concurrent requests. If you throw Promise.all(images.map(resize)) at it, you'll blast all 200 simultaneously, collect a pile of 429s, and watch your retry logic.

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.

Build an LRU cache from scratch: doubly linked list + hashmap for O(1) get and put, eviction by access order, and a bounded memoize wrapper on top.

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.

The naive 10-line debounce works for demos. Build the real thing: leading edges, trailing calls, cancel, flush, and a throttle that never silently drops the last event.

Call fib(45) naively and JavaScript will make over a billion recursive calls. Call it twice and you double that. Your CPU fan kicks on, the browser tab freezes, and the function you thought was fast becomes your performance bottleneck.

Picture this: a live dashboard that polls for new notifications every 3 seconds. The count increments in state, the UI updates, everything looks right — except the interval callback is silently reading count = 0 on every tick, because it.