20 Mistakes That Quietly Destroy JavaScript/TypeScript Codebases (Part 2)

Part 2 of a series on JS/TS codebase mistakes covers 9 practical pitfalls with before/after code examples. Topics include: mutating function parameters (use immutable returns instead), memory leaks from uncleared event listeners and timers, missing async cancellation with AbortController, fetch calls without timeouts, running independent async operations sequentially instead of with Promise.all, blocking the Node.js event loop with sync I/O or CPU-heavy work, timezone bugs with JavaScript's Date, writing tests that verify implementation details rather than behavior, and failing to validate API input at boundaries (enabling mass assignment attacks). Each mistake includes a concrete fix and production-level context.