zod-schema-validation

---

name: zod-schema-validation description: Best practices for Zod schema validation and type inference in TypeScript applications.

Zod Schema Validation

You are an expert in Zod schema validation and type inference for TypeScript applications.

Core Principles

• Utilize Zod for schema validation and type inference
• Validate data at system boundaries (API, forms, external data)
• Leverage TypeScript type inference from Zod schemas
• Implement early returns and guard clauses for validation errors

Schema Design

Basic Schema

import { z } from 'zod'

const UserSchema = z.object({
  id: z.string().uuid(),
  email: z.string().email(),
  name: z.string().min(1).max(100),
  age: z.number().int().positive().optional(),
  role: z.enum(['admin', 'user', 'guest']),
  createdAt: z.date(),
})

type User = z.infer<typeof UserSchema>

Best Practices

• Define schemas close to where they're used
• Use .infer to derive TypeScript types
• Compose schemas using .extend(), .merge(), .pick(), .omit()
• Create reusable base schemas for common patterns

Validation Patterns

Safe Parsing

const result = UserSchema.safeParse(data)
if (!result.success) {
  console.error(result.error.format())
  return
}
// result.data is typed as User

Transform and Refine

const schema = z.string()
  .transform((val) => val.trim().toLowerCase())
  .refine((val) => val.length > 0, 'Cannot be empty')

Form Integration

• Use Zod with react-hook-form via @hookform/resolvers/zod
• Define form schemas that match your form structure
• Handle validation errors in UI appropriately
• Use .partial() for optional update forms

API Validation

• Validate request bodies in API routes
• Validate query parameters and path params
• Return structured error responses
• Use discriminated unions for different response types

Error Handling

• Implement custom error messages for better UX
• Use .format() for structured error output
• Create custom error maps for i18n support
• Handle nested object errors appropriately

Advanced Patterns

Discriminated Unions

const ResultSchema = z.discriminatedUnion('status', [
  z.object({ status: z.literal('success'), data: UserSchema }),
  z.object({ status: z.literal('error'), message: z.string() }),
])

Recursive Schemas

const CategorySchema: z.ZodType<Category> = z.lazy(() =>
  z.object({
    name: z.string(),
    children: z.array(CategorySchema),
  })
)

Performance

• Precompile schemas that are used frequently
• Avoid creating schemas inside render functions
• Use .passthrough() or .strict() intentionally
• Consider partial validation for large objects