npm install zod

import { z } from 'zod';

// Define schema — becomes both runtime validator and TypeScript type
const UserSchema = z.object({
  id: z.string().uuid(),
  name: z.string().min(1).max(100),
  email: z.string().email(),
  age: z.number().int().min(0).max(150).optional(),
  role: z.enum(['admin', 'user', 'moderator']),
  createdAt: z.string().datetime(),
});

// Infer the TypeScript type — no duplication
type User = z.infer<typeof UserSchema>;

// Validate API response at runtime
async function fetchUser(id: string): Promise<User> {
  const res = await fetch(`/api/users/${id}`);
  const raw = await res.json();

  // Throws ZodError with detailed messages if shape is wrong
  return UserSchema.parse(raw);
}

// Safe parse for form validation — no throw
function validateForm(formData: unknown) {
  const result = UserSchema.safeParse(formData);

  if (!result.success) {
    // Structured field-level errors
    const errors = result.error.flatten().fieldErrors;
    console.log(errors);
    // { email: ['Invalid email'], name: ['String must contain at least 1 character'] }
    return { success: false, errors };
  }

  return { success: true, data: result.data };
}

// Composable — extend existing schemas
const CreateUserSchema = UserSchema.omit({ id: true, createdAt: true });
type CreateUserInput = z.infer<typeof CreateUserSchema>;

// Validate environment variables at startup
const EnvSchema = z.object({
  DATABASE_URL: z.string().url(),
  PORT: z.coerce.number().default(3000),
  NODE_ENV: z.enum(['development', 'production', 'test']),
});

const env = EnvSchema.parse(process.env);
// env.PORT is typed as number, not string