TypeScript Development Rules

Prerequisite Detection

Inspect tsconfig, runtime/framework configuration, lint/format configuration, path aliases, package scripts, and representative modules before applying project conventions. Treat a rule as project-specific only when configuration or an established pattern supports it. Label limited-pattern conclusions as inferred. When competing conventions change a public contract, runtime behavior, or error boundary, stop and name the source or user decision required.

Type Safety in Backend Implementation

Type Safety in Data Flow Input Layer (unknown) -> Type Guard -> Business Layer (Type Guaranteed) -> Output Layer (Serialization)

Backend-Specific Type Scenarios:

• API Communication: Receive responses as unknown and validate them with type guards
• Form Input: External input as unknown, type determined after validation
• Legacy Integration: Receive the legacy boundary as unknown; isolate any evidence-backed assertion in the adapter that owns the boundary
• Test Code: Define mock input/output types with the configured test harness; use Partial<T> for intentionally partial fixtures and typed vi.fn<[Args], Return>() only when Vitest is configured

Coding Conventions