typescript-rules

Type safety in TypeScript backend code is enforced by applying type guards to validate unknown inputs from APIs and databases, then propagating guaranteed types through business logic layers. Use the unknown type at entry points, implement validation functions, and maintain explicit types throughout the data flow to eliminate runtime errors.

Handle errors in TypeScript API endpoints using custom error classes for domain-specific exceptions, async/await patterns with explicit error propagation, and layer-specific error handling. Implement a Result type pattern to encode success and failure states, allowing upper layers to handle errors appropriately.

Validate external API responses by treating them as unknown type, implementing type guard functions to check structure and values, then converting validated data to your internal types. This ensures type safety when integrating with third-party APIs and legacy systems.

Yes, dependency injection works with TypeScript type safety through parameterization—pass dependencies as function parameters with explicit types rather than relying on globals or service locators. This maintains type guarantees and makes error handling and testing more straightforward.

Custom error classes enable structured, domain-specific error handling in backend services. They allow layer-specific error processing—catching and transforming errors appropriately as they propagate—and provide type-safe error context for logging and API responses.

Structure form input validation by treating raw input as unknown, applying type guard functions to validate structure and constraints, then converting to typed domain objects. This pattern applies to API request bodies, form submissions, and any untrusted external data.