A script I just made to convert lossless OBS .MKV recordings to compressed .MP4 files, also outputting per-track .MP3 files. Requires ffmpeg on path, as well as TeraCopy to move converted files.

recodeq.js

// Copyright (c) 2022 FelicitusNeko
// 
// This software is released under the MIT License.
// https://opensource.org/licenses/MIT

const { readdirSync, writeFileSync, unlinkSync } = require("fs");
const { spawn } = require("child_process");
const { join, basename } = require("path");

const convertTarget = "C:\\Path\\To\\ConvertedFiles";
const completeTarget = "C:\\Path\\To\\RawFiles";
const pathTeracopy = "C:\\Program Files\\TeraCopy\\TeraCopy.exe"
const fileList = join(__dirname, "file.list");

const spawnFfmpeg = (file, output) => new Promise((f, r) => {
  const fileIsArray = Array.isArray(file);
  if (output == null) {
    if (!fileIsArray) output = basename(file, ".mkv");
    else r(new Exception("Output pattern must be specified if array of files is provided"));
  }

  const params = [
    "-map", "0:v", "-map", "0:a:0", // Grab the video and primary audio track
    "-c:v", "libx264",
    "-preset", "veryslow",
    "-tune", "animation",
    "-crf", "20",
    "-max_muxing_queue_size", "1024", // TODO: maybe this can be automatically adjusted as needed
    "-c:a", "libmp3lame",
    "-q:a", "2",
    join(convertTarget, `${output}.mp4`),

    "-map", "0:a:1", "-vn", // Grab the mic track
    "-c:a", "libmp3lame",
    "-q:a", "2",
    join(convertTarget, `${output}-mic.mp3`),

    "-map", "0:a:2", "-vn", // Grab the game audio track
    "-c:a", "libmp3lame",
    "-q:a", "2",
    join(convertTarget, `${output}-game.mp3`),

    "-map", "0:a:3", "-vn", // Grab the guest track
    "-c:a", "libmp3lame",
    "-q:a", "2",
    join(convertTarget, `${output}-guest.mp3`)
  ];

  if (fileIsArray) params.unshift(
    "-protocol_whitelist", "pipe,file", // so that we can pipe in a concat list without creating it
    "-f", "concat",
    "-safe", "0", // because we're using full path defs, which ffmpeg doesn't like
    "-i", "-" // read stdin
  )
  else params.unshift("-i", file);

  var proc = spawn("ffmpeg.exe", params, {
    stdio: ["pipe", "inherit", "inherit"]
  });

  if (fileIsArray) {
    proc.stdin.write(file.map(i => `file '${join(__dirname, i)}'`).join("\n"));
    proc.stdin.end();
  }

  proc.on("close", code => code == 0 ? f() : r(code));
});

const spawnTeracopy = () => new Promise((f, r) => {
  const proc = spawn(pathTeracopy, [
    "Move",
    `*${fileList}`,
    completeTarget,
  ], { stdio: "ignore" });

  proc.on("close", code => code == 0 ? f() : r(code));
});

(async () => {
  const files = 
     readdirSync(__dirname, { withFileTypes: true })
    .filter(file => file.isFile() && file.name.endsWith(".mkv"))
    .sort((a, b) => a.name.localeCompare(b.name));
  const concats = {};

  for (const file of files) {
    const { name } = file, base = basename(name, ".mkv");
    const pattern = /^(.*)\.(\d+)/.exec(base);

    if (pattern != null) {
      const [_, combineName, part] = pattern;
      if (!concats[combineName]) concats[combineName] = [];
      concats[combineName][Number.parseInt(part)] = name;
    }
    else await spawnFfmpeg(name);
  }

  for (const combineName in concats) {
    const parts = concats[combineName].filter(i => i != null);
    await spawnFfmpeg(parts, combineName);
  }

  writeFileSync(fileList, files.map(file => join(__dirname, file.name)).join("\n"));
  await spawnTeracopy();
  unlinkSync(fileList);
})();