expo-aac-to-m4a

iOS-only Expo Native Module that remuxes an AAC/ADTS (.aac) audio file into an M4A/MP4 (.m4a) container — without re-encoding. Preserves original codec, sample rate, channels, and bitrate.

Uses Apple's AVFoundation / AVAssetExportSession with AVAssetExportPresetPassthrough. No FFmpeg. No third-party dependencies.

Requirements

| Requirement | Version | |---|---| | Expo SDK | ≥ 49 | | iOS | ≥ 13.4 | | Xcode | ≥ 14 | | EAS Build / Expo Dev Client | Required (bare workflow) |

Android is not supported. AVFoundation is Apple-only.

Installation

# npm
npm install expo-aac-to-m4a

# or yarn
yarn add expo-aac-to-m4a

Then rebuild your native project:

# Expo Dev Client (local)
npx expo run:ios

# EAS Build (cloud)
eas build --platform ios --profile development

No manual iOS project changes required — expo-module.config.json handles auto-linking.

API

convertAacToM4a(inputPath, outputPath?)

convertAacToM4a(inputPath: string, outputPath?: string): Promise<string>

| Parameter | Type | Description | |---|---|---| | inputPath | string | Absolute path to the source .aac file | | outputPath | string (optional) | Absolute path for the output .m4a. Defaults to same directory/name as input with .m4a extension |

Returns: Promise<string> — absolute path of the created .m4a file.

Throws: Rejects with a descriptive error if the input file is invalid, unreadable, or export fails.

Usage Example

import { convertAacToM4a } from 'expo-aac-to-m4a';
import * as FileSystem from 'expo-file-system';

async function handleConvert() {
  // Replace with the real path to your .aac file
  const inputPath = FileSystem.documentDirectory + 'recording.aac';

  try {
    // Option A — auto-derive output path (same folder, .m4a extension)
    const m4aPath = await convertAacToM4a(inputPath);
    console.log('M4A saved at:', m4aPath);

    // Option B — specify output path explicitly
    const customPath = FileSystem.documentDirectory + 'converted/audio.m4a';
    const m4aPath2 = await convertAacToM4a(inputPath, customPath);
    console.log('M4A saved at:', m4aPath2);
  } catch (error) {
    console.error('Conversion failed:', error);
  }
}

How It Works

.aac (ADTS raw stream)
        │
        ▼
  AVURLAsset (load)
        │
        ▼
  AVAssetExportSession
    preset  = AVAssetExportPresetPassthrough  ← no transcode
    fileType = .m4a                            ← MP4 container
        │
        ▼
  .m4a (MP4 container, same AAC stream inside)

The audio data is never decoded or re-encoded — only the container changes from a raw ADTS byte stream to an MPEG-4 (ISO Base Media) container. This is lossless and very fast.

Error Codes

| Code | Cause | |---|---| | ERR_DELETE_EXISTING | Could not remove a pre-existing file at outputPath | | ERR_EXPORT_SESSION | AVAssetExportSession could not be initialized (bad input file) | | ERR_EXPORT_FAILED | AVFoundation export failed (see error message for details) | | ERR_EXPORT_CANCELLED | Export was cancelled externally | | ERR_EXPORT_UNKNOWN | Unexpected export session status |

Project Structure

expo-aac-to-m4a/
├── src/
│   ├── index.ts                  # Public JS/TS API
│   └── ExpoAacToM4aModule.ts     # Native module binding
├── ios/
│   ├── ExpoAacToM4aModule.swift  # Swift implementation
│   └── ExpoAacToM4a.podspec      # CocoaPods spec
├── expo-module.config.json       # Expo auto-linking config
├── tsconfig.json
├── package.json
└── README.md

Publishing to npm

# 1. Build TypeScript
npm run build

# 2. Login to npm
npm login

# 3. Publish
npm publish --access public

License

MIT © Geet Purwar