Why does every query to the database cause the same error?

Question

MySQL

Good morning, please tell me why these characters appear in my line: \
I already tried replace, and splice doesn’t help, they don’t seem to exist, but they are there when it displays an error, I killed the night in search of a solution. Please help .

app.post('/register-step2', (req,res,next) => {
  let email = req.body.email;
  let role = req.body.role;
  let firstname = req.body.firstname;
  let password = req.body.password;

  let select = "SELECT * FROM users WHERE email = '" + email + "'";
  let insert = "INSERT INTO users (firstname,email,password,role) VALUES('" + firstname + "','" + email + "','" + hash + "','" + role + "')"

  connection.query(select, (err,res) => {

    if (res.length < 1) {

      bcrypt.genSalt(saltRounds, (err, salt) => {
    			bcrypt.hash(password, salt, (err, hash) => {

          connection.query(insert, (err,res) => {
          	
            	if(err) console.log(err);
            	
            	else {
            		console.log(email)
            	}

          });

        });
      });

    }

    else {

      return console.log('user registered!');

    }

  });

  return res.redirect('/step2')
});

code: 'ER_PARSE_ERROR',
errno: 1064,
sqlMessage:
'You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near \'\'Text)\' at line 1',
sqlState: '42000',
index: 0,
sql:
'INSERT INTO users (firstname,email,password,role) VALUES(\'History\',\'[email protected]\',\'$2b$10$0bfFB/gNQaMIDPKc34zq.em3/DRAgrHiO5YpkizNA7gfqt1ZkQcHW\',\'Text)' }

Answer 1

let select = `SELECT * FROM users WHERE email = '${email}'`;
let insert = `INSERT INTO users (firstname, email, hash, role) VALUES('${firstname}' ,'${email}', '${password}', '${role}')`