code: function useYarn(setYarn) {
    if (typeof setYarn !== "undefined") {
      allowYarn = setYarn;
    }
    if (typeof allowYarn !== "undefined") {
      return allowYarn;
    }
    return getYarnVersionIfAvailable() ? true : false;
  }
docs: Use Yarn if available, it's much faster than the npm client. Return the version of yarn installed on the system, null if yarn is not available.