TypeScript patterns for type-first development, making illegal states unrepresentable, and exhaustive handling.

• Use discriminated unions, branded types, and const assertions to encode business rules in the type system and prevent invalid states at compile time
• Validate at system boundaries with Zod schemas as single source of truth; infer TypeScript types automatically to keep types and validation in sync
• Enforce exhaustive handling with never checks in switch statements and default cases; unhandled code paths become compile errors
• Wrap external calls and async operations with contextual error messages; propagate failures rather than silently catching them
• Load and validate configuration at startup with Zod; access typed config object throughout the codebase instead of process.env directly