的Javascript用户名验证

Question

对于这个功课我必须写检查，如果被有效如下所示用户名密码，但如何将最后警告一下，如果前两个警示存在吗？

function isUserNameValid(user) { 
    "use strict"; 
    var alert = ""; 
    if (user.length > 12) { 
    alert("cannot be more than 12 letters long"); 
    return false; 
    } 
    alert = /^[A-Za-z]+$/; 
    if (!alert.test(user)) { 
    alert("must only use letters A-Z or a-z"); 
    return false; 
    } 
    alert = /^[A-Za-z]+$/;  //<--(this part is the question?) 
    if(user.length > 12 || if(!alert.test(user)) { 
    alert("must only use letters A-Z or a-z and cannot be more than 12 letters long)"; 
    return false; 
} 
return true; 
} 

Answer 1

把第一，如果下面和两个

if(user.length > 12 && if(!a.test(user)) { 
alert("must only use letters A-Z or a-z and cannot be more than 12 letters long)"; return false; 
}