`z.union` type inside `z.object` returns `(UnionType) & (UnionType | undefined)` when `z.array()` is one of the options

[anthonyma94]

When an array is one of the options in z.union inside z.object, the inferred type becomes (UnionType) & (UnionType | undefined) instead of just UnionType.

const UnionType = z.object({
  test: z.union([z.string(), z.number()])
});
type UnionType = z.infer<typeof UnionType> // { test: string | number }

const UnionTypeBug = z.object({
  test: z.union([z.string(), z.number().array()])
});
type UnionTypeBug = z.infer<typeof UnionTyptBug> // { test: (string | number[]) & (string | number[] | undefined) }, should be { test: string | number[] }

[ferjul17]

This is the same error as #2334

[6XGate]

This is the same error as #2334

Looks the other way around since this is the older issue 😄
But this is also looks like a dup of #2203.

[6XGate]

My work around for this, for my use case, was to just add a transform to ensure only a single output type exists. Ex:

export type ItemTextGetter = (item: unknown) => string
export type ItemValueGetter = (item: unknown) => unknown

type Items = z.infer<typeof Items>
const Items = z.array(z.unknown()).min(1).or(z.record(z.unknown()).refine(is.nonEmptyObject))
  .transform(v => Object.values(v))

type ItemTextSpecifier = z.infer<typeof ItemTextSpecifier>
const ItemTextSpecifier = z.string().min(1).or(z.custom<ItemTextGetter>(v => typeof v === 'function'))
  .transform(v => (typeof v !== 'function' ? (item: unknown) => String(get(item, v)) : v))

type ItemValueSpecifier = z.infer<typeof ItemValueSpecifier>
const ItemValueSpecifier = z.string().min(1).or(z.custom<ItemValueGetter>(v => typeof v === 'function'))
  .transform(v => (typeof v !== 'function' ? (item: unknown) => get(item, v) as unknown : v))

export type SelectDialogInputs = z.infer<typeof SelectDialogInputs>
export const SelectDialogInputs = z.object({
  items: Items,
  itemText: ItemTextSpecifier.default('text'),
  itemValue: ItemValueSpecifier.default('value')
})

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[6XGate]

Not sure any work has been done to fix this bot.

[Pyrolistical]

should be closed as duplicate of #2203

[dosubot]

This is a TypeScript type inference issue that has affected union types in Zod, especially when arrays are involved. Recent updates to Zod have addressed union type inference in object properties, ensuring that types from direct unions and object property unions are now inferred consistently. If you're seeing this issue, I recommend upgrading to the latest version of Zod 4, as a recent pull request specifically fixed union type inference in object properties (see PR).

If you still encounter this after upgrading, you can work around it by adding an explicit type annotation to your schema, but this can introduce type safety gaps if the annotation and schema diverge. If you’re still seeing the problem on the latest version, please share your Zod version and a minimal reproduction.

Let me know if this resolves your issue—if so, feel free to close the thread!

To reply, just mention @dosu.

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