Execute my functions Asynchronously in NodeJS

I am new to NodeJS. I know there are a lot of questions about asynchronous NodeJS but I couldn't find exactly what I am looking for.

My problem is: I want to check if username and email already exist or not in my database. Two separate functions for username and email. Another function is for storing data to database.

I don't know to do this using asynchronous NodeJS pattern.

User.js (mongoose Schema)

const mongoose = require('mongoose');
var userSchema = mongoose.Schema({
    name: String,
    username: { type: String, required: true, unique: true },
    password: { type: String, required: true },
    email: { type: String, required: true, unique: true},
    aiub_id: String,    
});
const Users = module.exports = mongoose.model('User', userSchema);

module.exports.addUser = function (user, callback) {
    user.save(callback);
}
module.exports.usernameExist = function (givenUsername, callback) {
    Users.find({ username: givenUsername }, callback);
}
module.exports.emailExist = function (givenEmail, callback) {
    Users.find({ username: givenEmail}, callback);
}

index.js (route)

route.post('/signup', function(req, res){
// GRAB USER INFO FROM HTML FORM
var newUser = new User({
    name : req.body.tfullName,
    username : req.body.tusername,
    password : req.body.tpassword,      
    email : req.body.temail,
    aiub_id : req.body.tuserID
});

// This block send 200 if username doesn't exist
User.usernameExist(newUser.username, function (err, result){
    if(err){
        throw err;
    }
    if(result.length <= 0){
        res.send({status : 200 });
    }else{
        res.send({status : 100 });
    }
});

});

Please me help to solve this and please forgive if it sounds stupid.

via Big Guy