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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.

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

`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.

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.

Build Node.js's EventEmitter from scratch in 100 lines. Learn why listener removal requires saved references, how once() wraps callbacks, and what that MaxListenersExceededWarning actually means.