const storage = {
    users: {},
    addOrUpdateUser: function(user) {
        this.users[user.id] = user
   },
   getUserById: function(id) {
       if(this.users[id]) return this.users[id]
       return null //или [], смотря как у вас принято
   }
}
module.exports = storage

var mysql = require('mysql');
const storage = require('./storage')

var conn = mysql.createConnection({
  database: '',
  host: "",
  user: "",
  password: ""
});

conn.connect(function(err) {
  if (err) throw err;
});

conn.query("SELECT username, id  FROM Users WHERE id ="+id,
  function(err, results, fields) {
      console.log(err);
      console.log(results[0].username);
      storage.addOrUpdateUser(results[0])
      console.log(storage.getUserById(results[0].id))
});