从node.js后端控制前端javascript

I am building a chat app and I want to ask users to input their username. JQuery front-end code slides a form into view (on ready), stores the data into variables and then loads the chat (when enter key or button is pressed). How do I stop that animation until I validate user input on server-side? I am using node.js for backend. Any ideas?I am building a chat app and I want to ask user