Zod for new TypeScript projects and shared frontend/backend schemas. express-validator for existing JavaScript Express projects with a tight integration preference. Both work; Zod is more modern and reusable.

You define a schema (z.object({ email: z.string().email() })) and use it anywhere: in a middleware, in a frontend form, in tests. The schema is the source of truth, not the validation call. This makes it reusable.

safeParse returns { success: true, data } or { success: false, error } instead of throwing. Useful in middleware where you want to handle errors gracefully. express-validator uses validationResult which is similar but less composable.