Unions

Zod includes a built-in z.union method for composing "OR" types.

const stringOrNumber = z.union([z.string(), z.number()]);
 
stringOrNumber.parse("foo"); // passes
stringOrNumber.parse(14); // passes

Zod will test the input against each of the "options" in order and return the first value that validates successfully.

For convenience, you can also use the .or method:

const stringOrNumber = z.string().or(z.number());

Optional string validation:

To validate an optional form input, you can union the desired string validation with an empty string literal.

This example validates an input that is optional but needs to contain a valid URL:

const optionalUrl = z.union([z.string().url().nullish(), z.literal("")]);
 
console.log(optionalUrl.safeParse(undefined).success); // true
console.log(optionalUrl.safeParse(null).success); // true
console.log(optionalUrl.safeParse("").success); // true
console.log(optionalUrl.safeParse("https://zod.dev").success); // true
console.log(optionalUrl.safeParse("not a valid url").success); // false

Discriminated unions

A discriminated union is a union of object schemas that all share a particular key.

type MyUnion = { status: "success"; data: string } | { status: "failed"; error: Error };

Such unions can be represented with the z.discriminatedUnion method. This enables faster evaluation, because Zod can check the discriminator key (status in the example above) to determine which schema should be used to parse the input. This makes parsing more efficient and lets Zod report friendlier errors.

With the basic union method, the input is tested against each of the provided "options", and in the case of invalidity, issues for all the "options" are shown in the zod error. On the other hand, the discriminated union allows for selecting just one of the "options", testing against it, and showing only the issues related to this "option".

const myUnion = z.discriminatedUnion("status", [
  z.object({ status: z.literal("success"), data: z.string() }),
  z.object({ status: z.literal("failed"), error: z.instanceof(Error) })
]);
 
myUnion.parse({ status: "success", data: "yippie ki yay" });

You can extract a reference to the array of schemas with the .options property.

myUnion.options; // [ZodObject<...>, ZodObject<...>]

To merge two or more discriminated unions, use .options with destructuring.

const A = z.discriminatedUnion("status", [
  /* options */
]);
const B = z.discriminatedUnion("status", [
  /* options */
]);
 
const AB = z.discriminatedUnion("status", [...A.options, ...B.options]);