index.js

const path = require('path')
const os = require('os')
const mcPath = require('minecraft-folder-path')

const launcherProfile = require(path.join(mcPath, 'launcher_profiles.json'))
const auth = launcherProfile.authenticationDatabase[launcherProfile.selectedUser]
const version = launcherProfile.profiles[launcherProfile.selectedProfile].lastVersionId

console.log(makeCmd({ version, auth, mcPath, clientToken: launcherProfile.clientToken }))

function makeCmd({version, auth, mcPath, clientToken}) { 
  const versionPath = path.join(mcPath, 'versions', version)
  const jar = path.join(versionPath, version + '.jar')
  const versionIndex = loadVersionIndex(version)

  const java = 'java'
  const javaArgs = ['-Xmx512M', '-Xmn128M']

  const args = versionIndex.minecraftArguments
    .replace('${version_name}', version)
    .replace('${game_directory}', mcPath)
    .replace('${auth_player_name}', auth.displayName)
    .replace('${auth_access_token}', auth.accessToken)
    .replace('${auth_session}', clientToken)
    .replace('${auth_uuid}', auth.uuid)
    .replace('${user_type}', 'legacy')
    .replace('${user_properties}', '{}')
    .replace('${version_type}', versionIndex.type)
    .replace('${assets_root}', path.join(mcPath, 'assets'))
    .replace('${assets_index_name}', versionIndex.assets)

  const libs = versionIndex.libraries.map(lib => {
    const split = lib.name.split(':')
    return path.join(mcPath, 'libraries', ...split[0].split("."), split[1], split[2],`${split[1]}-${split[2]}.jar`)
  })

  return [
    java,
    ...javaArgs,
    `"-Djava.library.path=${path.join(versionPath, version + '-natives')}"`,
    '-cp',
    `"${[...libs, jar].join(os.platform() === 'win32' ? ';' : ':')}"`,
    versionIndex.mainClass,
    args
  ].join(' ')
  
  function loadVersionIndex(version) {
    const versionPath = path.join(mcPath, 'versions', version)
    const versionIndex = require(path.join(versionPath, version))
    if (!versionIndex.inheritsFrom) return versionIndex
    versionIndex.libraries.push(...loadVersionIndex(versionIndex.inheritsFrom).libraries)
    return versionIndex
  }
}