Zod Schema Validation

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