Revision Sheet
Language Semantics
Coercion, destructuring, spread, optional chaining, and nullish coalescing are concise only when their edge cases are understood.
Defaulting and Data Shape
Safe defaults should preserve valid falsy values and should not hide broken required data contracts.
Async Coordination
Event emitters and async streams decouple producers and consumers, but they require cleanup, cancellation, error handling, and backpressure thinking.