Generate new username available, always getting Undefined - node.js

Question

I have a function to generate an available username, but always return undefined. I tried in many ways, in this case is a recursive function, but I'm always getting the same result, can you help me? Thanks
This is the code:
function generateNewUsernameAvailable(userName, number){
console.log('FUNCTION generateNewUsernameAvailable '+userName+' with number '+number);
User.countDocuments({ 'userName' : userName+number }, (err, count) => {
if (err) {
return `Error: ${err}`;
}
if (count == 0) {
return userName+number;
}else{
generateNewUsernameAvailable(userName, number+1);
}});
}
module.exports.existsUserName = function(req,res){
let userName = req.body.userName;
console.log('POST existsUserName '+userName);
User.countDocuments({ 'userName' : userName }, (err, count) => {
if (err) {
return res.status(500).send({message: `Error: ${err}`});
}
// Available
if (count == 0){
return res.status(200).send();
} else {
// Generate and return a new username available
console.log(generateNewUsernameAvailable('ricardo', 1));
res.status(400).send({message: 'newUsernameAvailable (Example ricardo1)'});
}
})
}

Answer 1

FindOne is faster than countDocuments/estimatedDocumentCount in this case. Both are promises, I'm going to add a possible solution:
function generateNewUsernameAvailable(userName, number){
return User
.findOne({ 'userName' : userName+number })
.then(function(result){
if (result){
return generateNewUsernameAvailable(userName, number+1);
}else{
return userName+number;
}
})
.catch(function(err) {
console.log(`Error: ${err}`);
throw err;
});
}
module.exports.existsUserName = function(req,res){
let userName = req.body.userName;
console.log('POST existsUserName '+userName);
User.countDocuments({ 'userName' : userName }, (err, count) => {
if (err) {
return res.status(500).send({message: `Error: ${err}`});
}
if (count == 0){
return res.status(200).send();
} else {
generateNewUsernameAvailable(userName,1)
.then(function(results){
return res.status(400).send({message: results});
})
.catch(function(err){
return res.status(500).send({message: `Error: ${err}`});
});
}
})
}