I'm trying to write a function that will take the email of the user as a parameter and return the first part of the email, up to but not including the "@" symbol. Problem is I'm terrible with functions, and there's something wrong with this function but I'm not sure what it is. When I try and write the function to the page to see if it worked correctly, it keeps showing up undefined.

function emailUsername(emailAddress)
{
    var userName = "";
    for(var index = 0; index < emailAddress.length; index++)
    {
        var CharCode = emailAddress.charCodeAt(index);
        if(CharCode = 64)
        {
            break;
        }
        else
        {
            userName += emailAddress.charAt(index);
            return userName;
        }
    }
}

var email = new String(prompt("Enter your email address: ",""));
var write = emailUsername(email);
document.write(write);

I'm sure there are other ways to do it but I need to follow roughly this format of using a function to check what's before the "@" and using methods to find it out.