Function check

packages/zod/src/v4/mini/schemas.ts:1774–1782

(fn: core.CheckFn<O>, params?: string | core.$ZodCustomParams)

// custom checks
// @__NO_SIDE_EFFECTS__
export function check<O = unknown>(fn: core.CheckFn<O>, params?: string | core.$ZodCustomParams): core.$ZodCheck<O> {
  const ch = new core.$ZodCheck({
    check: "custom",
    ...util.normalizeParams(params),
  });

  ch._zod.check = fn;
  return ch;
}

// ZodCustom
// custom schema