Trunc mail address in js [duplicate]

I need to remove what is after the @ in an email address in a javascript var like that: exemple@mail.com to exemple for example, how I can make that because I have seen the function

Math.trunc()

But to replace the user.DisplayName by the truncated email with in my code :

var user = user.DisplayName;
var email = user.email;
 if (user === null ) {
   //truncate email (user.email) 
   $(user).text(displayName);
  }
 else {
   //...
  }