The purpose of login function is to save the username and memo key to localStorage so that I can use it while sending account update data and auto-filling forms. But I should have done the username verification. Thank you for pointing it out.
I think SteemConnect login cannot help with the functionalities because we are changing authorities but it might be a way to verify if the user owns the username.
I am planning to learn unit testing and going to use it in future projects.
Thank you so much for the detailed review. :)