Make field required dependent on other field value

[knoefel]

I have the following schema:

export const schema = z
  .object({
    isOfferAvailable: z.boolean(),
    offerNotAvailableReason: z.string().optional(),
  })

So what i want here is, when isOfferAvailable is true, then offerNotAvailableReason should be required and also not be empty. I've read in other discussions here that i should use refine. But then i have to define the refinement on the root schema object and i can't set an error message on the offerNotAvailableReason field directly.

In yup there is when, where i can access another value from the schema, how can i achieve this with zod?

Sorry if this has been asked before, i searched through the discussions and issues and couldn't find anything which solved my problem. Thx in advance!!

[scotttrinh]

One way to approach this is to use an explicit discriminated union. Not sure if the requirement is that if it's true that it must be undefined or if you just ignore that, but here it is with an explicit undefined.

CodeSandbox

const offerIsAvailable = z.object({
  isOfferAvailable: z.literal(true),
  offerNotAvailableReason: z.undefined()
});
const offerNotAvailable = z.object({
  isOfferAvailable: z.literal(false),
  offerNotAvailableReason: z.string().min(1)
});
const schema = z.discriminatedUnion("isOfferAvailable", [
  offerIsAvailable,
  offerNotAvailable
]);

test("valid", (t) => {
  t.true(schema.safeParse({ isOfferAvailable: true }).success);
  t.true(
    schema.safeParse({
      isOfferAvailable: false,
      offerNotAvailableReason: "It's around here somewhere..."
    }).success
  );
  t.end();
});

test("invalid", (t) => {
  t.false(
    schema.safeParse({
      isOfferAvailable: true,
      offerNotAvailableReason: "Oh no!"
    }).success
  );
  t.end();
});