merge-vtt/index.js

A simple tool to merge multiple WebVTT (.vtt) files into a single file.

const fs = require("fs");
const path = require("path");

function mergeVTT(transcriptions) {
  let headerFound = false;
  let mergedTranscription = [];
  let timeOffset = 0;
  let cueCounter = 1;

  transcriptions.forEach((transcription) => {
    const lines = transcription.trim().split("\n");

    lines.forEach((line) => {
      if (line.trim() === "") {
        return;
      }

      if (line.trim() === "WEBVTT") {
        if (!headerFound) {
          mergedTranscription.push(line);
          headerFound = true;
        }
        return;
      }

      if (!isNaN(line)) {
        line = String(cueCounter++);
      }

      if (line.includes("-->")) {
        line = adjustTimestamps(line, timeOffset);
        mergedTranscription.push(line);
      } else {
        mergedTranscription.push(line);
      }
    });

    timeOffset = updateTimeOffset(lines, timeOffset);
  });

  return formatMergedTranscription(mergedTranscription);
}

function adjustTimestamps(line, offset) {
  const timestamps = line.split("-->");
  const start = convertTimestampToSeconds(timestamps[0].trim()) + offset;
  const end = convertTimestampToSeconds(timestamps[1].trim()) + offset;

  const newStart = convertSecondsToTimestamp(start);
  const newEnd = convertSecondsToTimestamp(end);

  return `${newStart} --> ${newEnd}`;
}

function convertTimestampToSeconds(timestamp) {
  const [hours, minutes, seconds] = timestamp.split(":").map(parseFloat);
  return hours * 3600 + minutes * 60 + seconds;
}

function convertSecondsToTimestamp(seconds) {
  const date = new Date(seconds * 1000).toISOString();
  return date.substr(11, 12);
}

function updateTimeOffset(lines, offset) {
  const lastTimestampLine = lines[lines.length - 2];
  const timestamps = lastTimestampLine.split("-->");
  const end = convertTimestampToSeconds(timestamps[1].trim());

  return offset + end;
}

function formatMergedTranscription(mergedTranscription) {
  return mergedTranscription
    .join("\n")
    .replace(
      /\n(\d{2}:\d{2}:\d{2}\.\d{3} --> \d{2}:\d{2}:\d{2}\.\d    {3})\n/g,
      "\n\n$1\n"
    );
}

// Export the function for use in other files
module.exports = { mergeVTT };