zod

Define schemas with specificity: Prefer concrete schema methods over loose ones.

• Use z.string().min(1) instead of bare z.string() for required non-empty strings
• Use z.number().int().nonnegative() instead of bare z.number() when appropriate
• Use z.enum() for known string unions
• Use z.literal() for exact values

Derive types from schemas: Always use z.infer<typeof Schema> to derive TypeScript types from Zod schemas. Do not define the type separately and the schema separately — the schema is the source of truth.

const MySchema = z.object({
  name: z.string().min(1),
  count: z.number().int().nonnegative(),
});
type My = z.infer<typeof MySchema>;

Choose the right parse method:

• .safeParse() for external or untrusted input (API payloads, user input, parsed documents) — check .success before using .data
• .parse() for internal guaranteed data where a thrown error indicates a programming bug

Colocate schema with type: Place the Zod schema in the same file as the type it defines. Export both the schema and the inferred type.

Test stubs: Use zocker with createStubFromSchema() from @carrot-fndn/shared/testing to generate test data from schemas. This ensures stubs always match the schema definition.

Schema composition: Use .extend(), .merge(), .pick(), .omit(), and .partial() to compose schemas from existing ones rather than duplicating field definitions.

Optional fields: Use z.optional() or .optional() for truly optional fields. Remember that exactOptionalPropertyTypes is enabled in tsconfig, so optional fields must be explicitly marked.

Const-object + z.enum() pattern: For value sets, define a const object mapping human-readable keys to values, then derive a z.enum() using the Object.values tuple cast. Use .extract() or .literal() to narrow for deeper schema layers.

const DOCUMENT_CATEGORY = {
  Methodology: 'methodology',
  Audit: 'audit',
  Certificate: 'certificate',
} as const;

const DocumentCategorySchema = z.enum(
  Object.values(DOCUMENT_CATEGORY) as [string, ...string[]],
);

// Narrower schema for a specific layer
const AuditOnlySchema = DocumentCategorySchema.extract(['audit']);
const MethodologyLiteralSchema = z.literal(DOCUMENT_CATEGORY.Methodology);