I threw the application into apk, an error connecting to the database, although it works in the browser, what should I do?

Question

MySQL

db = mysql.createConnection({
  host: 'localhost',
  //socketPath: "/var/run/mysqld/mysqld.sock", 
  user : 'root', 
  password : '',
  database : 'game'
});

db.connect((err) => {
  if(err) console.log(err);
  console.log("Mysql connected");
});

db.query('SELECT email FROM players WHERE email=?', [email], (err,result,fields) => {
   		if(err) console.log(err);
   		e_mail = fields[0];
   		console.log(JSON.stringify(e_mail));
   	});
   	
   	if(e_mail == undefined) {
   		socket.emit('warning');
      }
      else {
        console.log(e_mail + '/' + email);
        db.query("INSERT INTO players (nickname,email) VALUES('" + nick + "','" + email +  "')", (err,result) => {

     	if(err) console.log(err);

     	console.log("Insert");
  			});
      }

Answer 1

What and where did you put it?
Your connection to the database goes through localhost. Accordingly, nothing will connect if you do not have mysql installed on the device, or connect via an external IP for the DBMS