# DetectorFunction

**Kind:** Type

**Source:** [`src/middleware/language/language.ts`](https://github.com/honojs/hono/blob/main/src/middleware/language/language.ts#L194)

**Part of:** [Middleware](subsystem-src-middleware)

Type for detector functions

`DetectorFunction` defines a function that detects the language associated with an incoming request or execution context. Language middleware calls this function to select the language value used by downstream handlers.

## Definition

```ts
(c: Context, options: DetectorOptions) => string | undefined
```

## Diagram

```mermaid
graph LR
  Request[Incoming request] --> Detector[DetectorFunction]
  Detector --> Language[Detected language]
  Language --> Middleware[Language middleware]
  Middleware --> Handler[Downstream handler]
```

## Usage

```ts
import type { DetectorFunction } from "./middleware/language/language";

const detectLanguage: DetectorFunction = (request) => {
  const header = request.headers.get("accept-language");

  if (!header) {
    return "en";
  }

  return header.split(",")[0].trim();
};

// Pass detectLanguage to the language middleware configuration.
```

## AI Coding Instructions

- Keep detector functions focused on reading request or context data and returning a language value.
- Return a fallback language when the expected request data is missing.
- Parse headers defensively because language headers can contain multiple values and quality markers.
- Keep custom detector logic compatible with the parameter and return types defined by `DetectorFunction`.