# IsLiteralUnion

**Kind:** Type

**Source:** [`src/validator/utils.ts`](https://github.com/honojs/hono/blob/main/src/validator/utils.ts#L9)

Checks if T is a literal union type (e.g., 'asc' | 'desc')
that should be preserved in input types.
Returns true for union literals, false for single literals or wide types.

`IsLiteralUnion<T>` evaluates whether `T` is a union of literal values, such as `'asc' | 'desc'`. Validator input-type logic uses this result to preserve literal unions while treating single literals and widened types differently.

## Definition

```ts
[Exclude<T, undefined>] extends [Base] ? [Exclude<T, undefined>] extends [UnionToIntersection<Exclude<T, undefined>>] ? false : true : false
```

## Diagram

```mermaid
graph LR
  T[Input type T] --> Check{Is T a literal union?}
  Check -->|Yes| True[true: preserve input type]
  Check -->|No| False[false: handle as non-union or wide type]
```

## Usage

```ts
import type { IsLiteralUnion } from './validator/utils'

type SortDirection = 'asc' | 'desc'
type SingleDirection = 'asc'
type WideString = string

type IsSortDirection = IsLiteralUnion<SortDirection> // true
type IsSingleDirection = IsLiteralUnion<SingleDirection> // false
type IsWideString = IsLiteralUnion<WideString> // false
```

## AI Coding Instructions

- Use `IsLiteralUnion<T>` only in type-level conditional logic; it has no runtime value.
- Preserve unions of literal strings or numbers when this type evaluates to `true`.
- Do not treat a single literal, such as `'asc'`, as a literal union.
- Check widened types such as `string`, `number`, and `boolean` separately when changing validator input-type behavior.