code: function createBundle(log = logger.log.bind(logger)) {
    if (!npm.can.prepare()) {
      log('building ts-jest')
      npm.spawnSync(['-s', 'run', 'build'], { cwd: rootDir })
    }
    log('creating ts-jest bundle')
    const res = npm.spawnSync(['-s', 'pack'], { cwd: rootDir })
    return join(rootDir, res.stdout.toString().trim())
  }
docs: This will trigger the build as well (not using yarn since yarn pack is bugy) Except that on npm < 4.0.0 the prepare doesn't exists