# StringLiteralUnion

**Kind:** Type

**Source:** [`src/utils/types.ts`](https://github.com/honojs/hono/blob/main/src/utils/types.ts#L120)

**Part of:** [Utils](subsystem-src-utils)

String literal types with auto-completion

`StringLiteralUnion` combines a set of string literal values with `string` while preserving editor autocomplete for the known literals. It is used when an API accepts predefined string options but must also allow custom string values.

## Definition

```ts
T | (string & Record<never, never>)
```

## Diagram

```mermaid
graph LR
  A[Known string literals] --> C[StringLiteralUnion]
  B[Custom string values] --> C
  C --> D[Autocomplete for known values]
  C --> E[Accepts any string]
```

## Usage

```ts
import type { StringLiteralUnion } from "../utils/types";

type Theme = StringLiteralUnion<"light" | "dark">;

const defaultTheme: Theme = "light";
const customTheme: Theme = "system";
```

## AI Coding Instructions

- Use `StringLiteralUnion` for string options that have known values and allow arbitrary custom values.
- Keep the known options as string literal unions, such as `"light" | "dark"`.
- Do not replace this type with `string` when autocomplete for known values is expected.
- Use the type in public option and configuration definitions where custom string extensions are valid.