How to Convert SRT to WebVTT: In-Browser Timecode & Syntax Guide

/**
 * Converts raw SubRip (.srt) subtitle string to WHATWG-compliant WebVTT (.vtt).
 * Pure client-side runtime with zero third-party dependencies.
 *
 * @param {string} srtText - Input SRT payload.
 * @returns {string} Fully compliant WebVTT data stream.
 */
export function convertSrtToVtt(srtText) {
  if (!srtText || typeof srtText !== 'string') {
    return 'WEBVTT\n\n';
  }

  // 1. Normalize line endings to standard Unix LF (\n)
  let normalized = srtText.replace(/\r\n|\r/g, '\n').trim();

  // 2. Normalize timestamp formatting:
  //    Matches '00:00:00,000 --> 00:00:00,000' and replaces comma with dot
  //    Preserves normal commas present in dialogue subtitles
  const timestampRegex = /(\d{2}:\d{2}:\d{2}),(\d{3})\s*(-->)\s*(\d{2}:\d{2}:\d{2}),(\d{3})/g;
  normalized = normalized.replace(timestampRegex, '$1.$2 $3 $4.$5');

  // 3. Transform legacy SRT <font color="..."> tags into WebVTT <c> tags
  normalized = normalized.replace(/<font color=["']?(#[A-Fa-f0-9]{3,6}|[a-zA-Z]+)["']?>/gi, '<c>');
  normalized = normalized.replace(/<\/font>/gi, '</c>');

  // 4. Prepend compulsory WebVTT magic sequence
  return `WEBVTT - Converted with ToolAcre Zero-Egress Engine\n\n${normalized}\n`;
}

// 5. Direct Browser Blob File Downloader helper
export function triggerVttDownload(vttContent, filename = 'subtitles.vtt') {
  const blob = new Blob([vttContent], { type: 'text/vtt;charset=utf-8' });
  const downloadUrl = URL.createObjectURL(blob);
  const anchor = document.createElement('a');
  anchor.href = downloadUrl;
  anchor.download = filename;
  document.body.appendChild(anchor);
  anchor.click();
  document.body.removeChild(anchor);
  URL.revokeObjectURL(downloadUrl);
}