Request Validation and Typed JSON Responses with Zod
Parse and validate request bodies and query params, returning typed, well-structured error responses.
Why Validate at the Edge?
In Next.js 15 Route Handlers (app/api/.../route.ts), the request body is just untyped JSON. TypeScript types vanish at runtime, so a client can send anything.
Zod lets you describe the expected shape once and get both:
- A runtime check that rejects bad input.
- A static type inferred from the schema, so your handler code is fully typed.
This lesson builds a typed POST handler that validates the body and query params, then returns clean, structured JSON errors.
Defining a Schema
Start by declaring the shape you expect. Zod schemas double as the single source of truth for both validation and types.
Use z.infer to derive a TypeScript type from the schema. There is no duplication: change the schema and the type updates automatically.
import { z } from 'zod';
export const CreateUserSchema = z.object({
name: z.string().min(1, 'Name is required'),
email: z.string().email('Invalid email address'),
age: z.number().int().positive().optional(),
});
// Inferred type — fully typed, no duplication
export type CreateUserInput = z.infer<typeof CreateUserSchema>;All lessons in this course
- Designing RESTful Route Handlers with the Web Request API
- Node Runtime vs Edge Runtime Tradeoffs
- Streaming Responses and ReadableStream in Handlers
- Request Validation and Typed JSON Responses with Zod