How to use the tinode-sdk.credential function in tinode-sdk

doLogin(login, password, cred) {
    if (this.tinode.isAuthenticated()) {
      // Already logged in. Go to default screen.
      HashNavigation.navigateTo('');
      return;
    }
    // Sanitize and package credentail.
    cred = Tinode.credential(cred);
    // Try to login with login/password. If they are not available, try token. If no token, ask for login/password.
    let promise = null;
    let token = this.tinode.getAuthToken();
    if (login && password) {
      this.setState({password: null});
      promise = this.tinode.loginBasic(login, password, cred);
    } else if (token) {
      promise = this.tinode.loginToken(token.token, cred);
    }

    if (promise) {
      promise.then((ctrl) => {
        if (ctrl.code >= 300 && ctrl.text === 'validate credentials') {
          this.setState({loadSpinnerVisible: false});
          if (cred) {
            this.handleError("Code does not match", 'warn');

.then(() => {
        return this.tinode.createAccountBasic(login_, password_,
          {public: public_, tags: tags_, cred: Tinode.credential(cred_)});
      }).then((ctrl) => {
        if (ctrl.code >= 300 && ctrl.text == 'validate credentials') {