React apps slow down for predictable reasons: unnecessary re-renders, missing memoization, unvirtualized long lists, and oversized bundles. Every technique to fix them — with when to use each.

Async/await looks simple but hides deadly traps — sequential execution when you meant parallel, uncaught rejections, missing AbortController, await in forEach. Every pattern and pitfall covered.

JavaScript Proxy intercepts fundamental object operations at runtime. Validation, auto-logging, memoization, observable state, and negative array indexing — all implementable in under 20 lines each.

TypeScript generics go far beyond simple type parameters. Conditional types, the infer keyword, mapped types, and template literal types let you make illegal states unrepresentable.

Generator functions produce values on demand instead of computing everything upfront. They enable infinite sequences, lazy pipelines, and async iteration patterns that no other JavaScript feature can match.

WeakMap and WeakRef let objects be garbage collected even when referenced. They solve cache invalidation, DOM cleanup, and observer patterns without memory leaks. Here is how they actually work.