How to use the buttercup.FileDatasource function in buttercup

.then(ans => {
                    const datasource = new Buttercup.FileDatasource(bcupPath);
                    datasource.load(ans.master_password).then((archive) => {
                    // This is only guaranteed to work on buttercup 0.14.0, awaiting PR in buttercup
                        const groups = archive.getGroups();
                        const group = groups.filter((g) => g._remoteObject.title === 'git-labelmaker')[0];
                        const token = group.getAttribute('token');
                        res(token);
                    })
                  .catch(e => {
                      if (e.message === 'Failed opening archive: Error: Encrypted content has been tampered with') {
                          return rej({
                              id: 'PASSWORD',
                          });
                      }
                      return rej(err(e.message));
                  });
                })

save: function(datasourcePath, password) {
            var archive = Buttercup.Archive.createWithDefaults(),
                datasource = new Buttercup.FileDatasource(datasourcePath);

            return datasource.save(archive, password);
        }
    };

const writeToken = (password, token) => new Promise((res, rej) => {
    const datasource = new Buttercup.FileDatasource(bcupPath);
    if (!datasource) return rej('No datasource');
    const archive = Buttercup.Archive.createWithDefaults();
    const group = archive.createGroup('git-labelmaker');
    group.setAttribute('token', token);
    datasource.save(archive, password);
    return res(token);
});

load: function(datasourcePath, password) {
            var workspace = new Buttercup.Workspace(),
                datasource = new Buttercup.FileDatasource(datasourcePath);

            return datasource.load(password).then(function(archive) {
                workspace
                    .setArchive(archive)
                    .setDatasource(datasource)
                    .setPassword(password);

                return workspace;
            });
        },