var conn = new jsforce.Connection({accessToken: getCookie('sid')});

conn.metadata.read('Profile', profile_name, function (err, metadata) {
  if (err) {
    return console.error(err);
  }

  // set the permissions you want in the metadata object

  conn.metadata.update('Profile', metadata, function (err, results) {
    if (err) {
      return console.error(err);
    }

    console.log('Done');
  });
});