Async Concurrency Control in Node.js: Semaphores, Throttling, and Backpressure

Promise.all(items.map(fn)) is one of the most dangerous lines in a Node.js codebase. It looks reasonable — process everything in parallel, await the result. What it actually does is fire every promise simultaneously with zero regard for memory, downstream API rate limits, or database connection pools. At 50 items it's fine. At 50,000 it's an incident.

Controlling concurrency is a first-class concern in production Node.js services, not an afterthought. The patterns are simple to implement and the payoff is enormous.