typescript-strict

Strict TypeScript

Types must model the domain and fail early. Compile-time does not replace validation at the boundary. Form UX: better-forms.

Rules

• Treat strict as the baseline. Do not turn off flags to “pass the build”.
• Forbid new any . Prefer unknown + narrowing.
• Avoid blind as ; use type guards, predicates, and schemas.
• Model nullability explicitly (T | null), not with magic sentinels.
• Keep domain types in dedicated modules; do not spread duplicate interfaces.

Modeling

• Use union types and discriminated unions for states.
• Prefer Readonly / as const for immutable data.
• Generics: constrain with extends; avoid decorative generics.
• Extract types from values (typeof, ReturnType, Awaited) instead of duplicating.
• For external APIs, define request/response types and equivalent Zod/Valibot schemas.

Runtime

• Validate at the boundary: HTTP, forms, env, webhooks, localStorage.
• Inference: z.infer<typeof Schema> as a single source.
• Typed errors: result types or discriminable error classes.
• Env: parse with a schema at boot; fail if invalid.

Safe refactoring

1. Introduce types at the boundary.
2. Push unknown inward until narrowing.
3. Remove casts gradually.
4. Run tsc --noEmit / project typecheck after changes.

Anti-patterns

• any, as any, @ts-ignore without a short, local justification
• Giant interfaces with too many optional fields
• Fragile numeric enums when a union of strings is enough
• Typing only on the frontend while the backend returns any
• Lying to the compiler instead of fixing the model

Completion criteria

• No unjustified any in the areas touched
• Boundaries validated at runtime
• Discriminated unions for complex states
• Project typecheck passing
• Types readable and aligned with the domain