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You have a UserService that needs a Database. Your OrderService needs UserService, Database, and a PaymentGateway. Your NotificationService needs EmailClient and UserService.

The Two Sum problem is the "Hello World" of hash map problems. Most people solve it, move on, and treat it as a warmup. That's a mistake. The pattern behind Two Sum — trading space for lookup speed — shows up in dozens of harder problems.

Build a pub/sub system from 60 lines of JavaScript: subscribe, publish, unsubscribe, once(), namespace wildcards, error isolation, and async publish.

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?

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.

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.