How to Build a Flac To Mp3 Converter

// Server-side: FFmpeg conversion with metadata preservation
  import { exec } from 'child_process';
  import { promisify } from 'util';
  const execAsync = promisify(exec);

  interface ConvertOptions {
  bitrate: 128 | 192 | 256 | 320;
  vbr?: boolean;       // variable bitrate for better quality/size ratio
  vbrQuality?: number; // 0 (best) to 9 (worst) for VBR mode
  sampleRate?: number; // default: match source
  }

  async function convertFlacToMp3(
  inputPath: string,
  outputPath: string,
  options: ConvertOptions = { bitrate: 320 }
  ): Promise<void> {
  let command: string;

  if (options.vbr) {
    // Variable bitrate: better quality per byte
    const quality = options.vbrQuality ?? 2; // V2 ~ 190kbps, transparent for most
    command = `ffmpeg -i "${inputPath}" -codec:a libmp3lame -qscale:a ${quality} "${outputPath}"`;
  } else {
    // Constant bitrate
    command = `ffmpeg -i "${inputPath}" -codec:a libmp3lame -b:a ${options.bitrate}k "${outputPath}"`;
  }

  // -map_metadata 0 preserves FLAC tags (artist, album, etc.)
  command = command.replace('ffmpeg', 'ffmpeg -map_metadata 0');

  await execAsync(command);
  }

  // Browser-side: FFmpeg WASM
  async function convertInBrowser(flacFile: File, bitrate: number = 320): Promise<Blob> {
  const { createFFmpeg, fetchFile } = await import('@ffmpeg/ffmpeg');
  const ffmpeg = createFFmpeg({ log: false });
  await ffmpeg.load();

  ffmpeg.FS('writeFile', 'input.flac', await fetchFile(flacFile));
  await ffmpeg.run(
    '-i', 'input.flac',
    '-codec:a', 'libmp3lame',
    '-b:a', `${bitrate}k`,
    '-map_metadata', '0',
    'output.mp3'
  );

  const data = ffmpeg.FS('readFile', 'output.mp3');
  return new Blob([data.buffer], { type: 'audio/mpeg' });
  }