user.ts

  register(username, password, attr) {
    let attributes = [];

    for (var x in attr) {
      attributes.push(this.cognito.makeAttribute(x, attr[x]));
    }
    
    return new Promise((resolve, reject) => {
      this.cognito.getUserPool().signUp(username, password, attributes, null, function(err, result) {
        if (err) { 
          reject(err);
        } else {
          resolve(result.user);
        }
      });
    });
  }