title: Username already exists, please check for available options during sign up
body: <p>I am creating an angular app in which a user can register. But while registering I request him for some details like email, mobile number and all. Also, a username which he can choose. But here I want to do a check on the username as soon as he entered and display an error like," this username already exists in our database".</p>
  
  <p>I have mysql database and front end with angular 4. 
  I am new to this angular development, please help me how can i do this?</p>
  
  <p>Here is a sample image for google sign up,
  <a href="https://i.stack.imgur.com/FZGq5.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/FZGq5.png" alt="enter image description here"></a></p>