How can I stop a form from being submitted until it's validated?

Question

AJAX

There is a registration form, with it Ajax check for employment of the login from the database.
Ajax validation works fine, but the form is submitted anyway.
How to disable form submission if username is busy?
_______________________________________________________________
I'll send you the code if needed.

Answer 1

document.querySelector("#myForm").addEventListener("submit", function(e){
    if(!isValid){
        e.preventDefault();    //форма не отправится 
    }
});