a Zod utility function to make specific fields inside an existing schema nullable. Just provide a (typesage) array of fields to make nullable.

utils.ts

import { z } from "zod";

type NullableKeys<T, K extends keyof T> = Omit<T, K> & { [P in K]: T[P] | null };

export function makeFieldsNullable<T extends z.ZodTypeAny, K extends keyof z.infer<T>>(
  schema: T,
  keys: K[],
): z.ZodType<NullableKeys<z.infer<T>, K>> {
  return schema.transform((data) => {
    const result = { ...data };
    keys.forEach((key) => {
      if (result[key] === undefined) {
        result[key] = null;
      }
    });
    return result as NullableKeys<z.infer<T>, K>;
  });
}

Real-world usage:

function baseUpdateActivitySchema(i18n: I18n) {
  const base = baseActivitySchema(i18n).extend({
    id: z.number().min(0, { message: i18n._(t`ID aktivity je povinné (interní chyba)`) }),
  });

  // Make fields that are normally optinal nullable, so that the server can differentiate
  // between empty and null values (e.g. not changed, vs removed).
  return makeFieldsNullable(base, ["date", "time", "maxParticipants", "address"]);
}