--- name: zod-validation-expert description: "Expert in Zod — TypeScript-first schema validation. Covers parsing, custom errors, refinements, type inference, and integration with React Hook Form, Next.js, and tRPC." risk: safe source: community date_added: "2026-03-05" --- # Zod Validation Expert You are a production-grade Zod expert. You help developers build type-safe schema definitions and validation logic. You master Zod fundamentals (primitives, objects, arrays, records), type inference (`z.infer`), complex validations (`.refine`, `.superRefine`), transformations (`.transform`), and integrations across the modern TypeScript ecosystem (React Hook Form, Next.js API Routes / App Router Actions, tRPC, and environment variables). ## When to Use This Skill - Use when defining TypeScript validation schemas for API inputs or forms - Use when setting up environment variable validation (`process.env`) - Use when integrating Zod with React Hook Form (`@hookform/resolvers/zod`) - Use when extracting or inferring TypeScript types from runtime validation schemas - Use when writing complex validation rules (e.g., cross-field validation, async validation) - Use when transforming input data (e.g., string to Date, string to number coercion) - Use when standardizing error message formatting ## Core Concepts ### Why Zod? Zod eliminates the duplication of writing a TypeScript interface *and* a runtime validation schema. You define the schema once, and Zod infers the static TypeScript type. Note that Zod is for **parsing, not just validation**. `safeParse` and `parse` return clean, typed data, stripping out unknown keys by default. ## Schema Definition & Inference ### Primitives & Coercion ```typescript import { z } from "zod"; // Basic primitives const stringSchema = z.string().min(3).max(255); const numberSchema = z.number().int().positive(); const dateSchema = z.date(); // Coercion (automatically casting inputs before validation) // Highly useful for FormData in Next.js Server Actions or URL queries const ageSchema = z.coerce.number().min(18); // "18" -> 18 const activeSchema = z.coerce.boolean(); // "true" -> true const dobSchema = z.coerce.date(); // "2020-01-01" -> Date object ``` ### Objects & Type Inference ```typescript const UserSchema = z.object({ id: z.string().uuid(), username: z.string().min(3).max(20), email: z.string().email(), role: z.enum(["ADMIN", "USER", "GUEST"]).default("USER"), age: z.number().min(18).optional(), // Can be omitted website: z.string().url().nullable(), // Can be null tags: z.array(z.string()).min(1), // Array with at least 1 item }); // Infer the TypeScript type directly from the schema // No need to write a separate `interface User { ... }` export type User = z.infer; ``` ### Advanced Types ```typescript // Records (Objects with dynamic keys but specific value types) const envSchema = z.record(z.string(), z.string()); // Record // Unions (OR) const idSchema = z.union([z.string(), z.number()]); // string | number // Or simpler: const idSchema2 = z.string().or(z.number()); // Discriminated Unions (Type-safe switch cases) const ActionSchema = z.discriminatedUnion("type", [ z.object({ type: z.literal("create"), id: z.string() }), z.object({ type: z.literal("update"), id: z.string(), data: z.any() }), z.object({ type: z.literal("delete"), id: z.string() }), ]); ``` ## Parsing & Validation ### parse vs safeParse ```typescript const schema = z.string().email(); // ❌ parse: Throws a ZodError if validation fails try { const email = schema.parse("invalid-email"); } catch (err) { if (err instanceof z.ZodError) { console.error(err.issues); } } // ✅ safeParse: Returns a result object (No try/catch needed) const result = schema.safeParse("user@example.com"); if (!result.success) { // TypeScript narrows result to SafeParseError console.log(result.error.format()); // Early return or throw domain error } else { // TypeScript narrows result to SafeParseSuccess const validEmail = result.data; // Type is `string` } ``` ## Customizing Validation ### Custom Error Messages ```typescript const passwordSchema = z.string() .min(8, { message: "Password must be at least 8 characters long" }) .max(100, { message: "Password is too long" }) .regex(/[A-Z]/, { message: "Password must contain at least one uppercase letter" }) .regex(/[0-9]/, { message: "Password must contain at least one number" }); // Global custom error map (useful for i18n) z.setErrorMap((issue, ctx) => { if (issue.code === z.ZodIssueCode.invalid_type) { if (issue.expected === "string") return { message: "This field must be text" }; } return { message: ctx.defaultError }; }); ``` ### Refinements (Custom Logic) ```typescript // Basic refinement const passwordCheck = z.string().refine((val) => val !== "password123", { message: "Password is too weak", }); // Cross-field validation (e.g., password matching) const formSchema = z.object({ password: z.string().min(8), confirmPassword: z.string() }).refine((data) => data.password === data.confirmPassword, { message: "Passwords don't match", path: ["confirmPassword"], // Sets the error on the specific field }); ``` ### Transformations ```typescript // Change data during parsing const stringToNumber = z.string() .transform((val) => parseInt(val, 10)) .refine((val) => !isNaN(val), { message: "Not a valid integer" }); // Now the inferred type is `number`, not `string`! type TransformedResult = z.infer; // number ``` ## Integration Patterns ### React Hook Form ```typescript import { useForm } from "react-hook-form"; import { zodResolver } from "@hookform/resolvers/zod"; import { z } from "zod"; const loginSchema = z.object({ email: z.string().email("Invalid email address"), password: z.string().min(6, "Password must be 6+ characters"), }); type LoginFormValues = z.infer; export function LoginForm() { const { register, handleSubmit, formState: { errors } } = useForm({ resolver: zodResolver(loginSchema) }); const onSubmit = (data: LoginFormValues) => { // data is fully typed and validated console.log(data.email, data.password); }; return ( ); } ``` ### Next.js Server Actions ```typescript "use server"; import { z } from "zod"; // Coercion is critical here because FormData values are always strings const createPostSchema = z.object({ title: z.string().min(3), content: z.string().optional(), published: z.coerce.boolean().default(false), // checkbox -> "on" -> true }); export async function createPost(prevState: any, formData: FormData) { // Convert FormData to standard object using Object.fromEntries const rawData = Object.fromEntries(formData.entries()); const validatedFields = createPostSchema.safeParse(rawData); if (!validatedFields.success) { return { errors: validatedFields.error.flatten().fieldErrors, }; } // Proceed with validated database operation const { title, content, published } = validatedFields.data; // ... return { success: true }; } ``` ### Environment Variables ```typescript // Make environment variables strictly typed and fail-fast import { z } from "zod"; const envSchema = z.object({ DATABASE_URL: z.string().url(), NODE_ENV: z.enum(["development", "test", "production"]).default("development"), PORT: z.coerce.number().default(3000), API_KEY: z.string().min(10), }); // Fails the build immediately if env vars are missing or invalid const env = envSchema.parse(process.env); export default env; ``` ## Best Practices - ✅ **Do:** Co-locate schemas alongside the components or API routes that use them to maintain separation of concerns. - ✅ **Do:** Use `z.infer` everywhere instead of maintaining duplicate TypeScript interfaces manually. - ✅ **Do:** Prefer `safeParse` over `parse` to avoid scattered `try/catch` blocks and leverage TypeScript's control flow narrowing for robust error handling. - ✅ **Do:** Use `z.coerce` when accepting data from `URLSearchParams` or `FormData`, and be aware that `z.coerce.boolean()` converts standard `"false"`/`"off"` strings unexpectedly without custom preprocessing. - ✅ **Do:** Use `.flatten()` or `.format()` on `ZodError` objects to easily extract serializable, human-readable errors for frontend consumption. - ❌ **Don't:** Rely exclusively on `.partial()` for update schemas if field types or constraints differ between creation and update operations; define distinct schemas instead. - ❌ **Don't:** Forget to pass the `path` option in `.refine()` or `.superRefine()` when performing object-level cross-field validations, otherwise the error won't attach to the correct input field. ## Troubleshooting **Problem:** `Type instantiation is excessively deep and possibly infinite.` **Solution:** This occurs with extreme schema recursion (e.g. deeply nested self-referential schemas). Use `z.lazy(() => NodeSchema)` for recursive structures and define the base TypeScript type explicitly instead of solely inferring it. **Problem:** Empty strings pass validation when using `.optional()`. **Solution:** `.optional()` permits `undefined`, not empty strings. If an empty string means "no value," use `.or(z.literal(""))` or preprocess it: `z.string().transform(v => v === "" ? undefined : v).optional()`.

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