# reverse/reverse

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_(no submodules)_
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## Purpose

Provides `getMimeExtension` for reverse lookup: given a MIME type string, returns the canonical file extension associated with that type. This is the inverse of extension-based detection — useful for generating filenames, setting `Content-Disposition` headers, and file download naming.

## Features

- Returns the canonical (first) extension for any registered MIME type
- Returns `undefined` for unknown MIME types (no throws)
- Case-insensitive MIME type matching (delegates to registry)
- Returns extensions without a leading dot
- O(1) lookup via the registry's forward map

## Usage: Basic Example

```typescript
import { getMimeExtension } from "@suaveplan/mime/reverse/reverse";

console.log(getMimeExtension("image/png"));         // "png"
console.log(getMimeExtension("application/pdf"));   // "pdf"
console.log(getMimeExtension("text/javascript"));   // "js"
console.log(getMimeExtension("unknown/type"));      // undefined
```

## Usage: Advanced Example

```typescript
import { getMimeExtension } from "@suaveplan/mime/reverse/reverse";

function buildFilename(baseName: string, mimeType: string): string {
    const ext = getMimeExtension(mimeType);
    if (!ext) return baseName; // Unknown type — keep original name
    // Avoid double extension
    return baseName.endsWith(`.${ext}`) ? baseName : `${baseName}.${ext}`;
}

// Content-Disposition header generation
function contentDisposition(mimeType: string, name: string): string {
    const ext = getMimeExtension(mimeType);
    const filename = ext ? `${name}.${ext}` : name;
    return `attachment; filename="${filename}"`;
}

console.log(contentDisposition("application/pdf", "report"));
// 'attachment; filename="report.pdf"'
```

## API Reference

### `getMimeExtension(mime: string): string | undefined`

| Parameter | Type | Description |
|-----------|------|-------------|
| `mime` | `string` | MIME type string (e.g., `"image/png"`). |

**Returns:** The canonical file extension string without a leading dot, or `undefined` if the MIME type is not in the database.

**Throws:** Never.

## Implementation Notes

The "canonical" extension is the first element of the `extensions` array in the `MimeEntry`. The built-in database places the most common extension first by convention (e.g., `"jpeg"` before `"jpg"` for `image/jpeg`). Callers that need all extensions for a type should use `getEntryByMime` directly from `@suaveplan/mime/db/registry` and access `entry.extensions`.

### Lookup Tables

The reverse index maps MIME string → preferred extension, MIME string → alias list, and alias → canonical MIME. Tables are precomputed at module load from the same curated IANA database the forward lookup uses, so the two directions can never drift. Lookups are O(1) against frozen `Map` instances with zero runtime cost after first import.