Mongo username and email unique fields validation - mongodb

Question

Hi I want to tell the user which one is already taken if is Username or Email but the way Im validating this is in the save function if this finds a duplicate it will give you a error 11000 but I cant specify which one is already taken. I want the error to give an index or something I can put in a if statement explaining which one is the duplicate key so I can be more specific with the error. Is there a way i can accomplish this? Also why Im getting always index: 0? Shouldn't be different for each field? Let me know if you have any questions.
Schema
var user = db.Schema({
name: { type: String, required: true},
username: { type: String, required: true, index: { unique: true }},
email: { type: String, required: true, index: { unique: true }},
password: { type: String, required: true, select: false},
admin: { type: Boolean, required: true},
verify: { type: Boolean, required: true},
created_at: { type: String, required: true, default: Date.now },
updated_at: { type: String, required: true, default: Date.now },
campaigns_donated: []
})
Post Action
router.post('/register', function(req, res){
var user = new User()
user.name = req.body.name
user.username = req.body.username
user.email = req.body.email
user.password = req.body.password
user.admin = false,
user.verify = false
user.save(function(err) {
if (err) {
console.log(err);
if (err.code == 11000) {
return res.json({ success: false, message: 'username or email already taken'})
} else {
return res.send(err);
}
}
res.json({ success: true, message: 'User created'})
})
})
Pre Save Function
user.pre('save', function(next) {
var user = this
// hash the password only if the password has been changed or user is new
if (!user.isModified('password')) { return next() }
//Generate Hash function
bcrypt.hash(user.password, 10, function(err, hash) {
if (err) return next(err)
//Change Password to hash
user.password = hash
next()
})
})
Console log error
{ [MongoError: E11000 duplicate key error index: ReachPeeps.users.$username_1 dup key: { : "xandor" }]
name: 'MongoError',
message: 'E11000 duplicate key error index: ReachPeeps.users.$username_1 dup key: { : "xandor" }',
index: 0,
code: 11000,
errmsg: 'E11000 duplicate key error index: ReachPeeps.users.$username_1 dup key: { : "xandor" }' }