node.js Global connection already exists. Call sql.close() first

You have to create a poolConnectiontry this:

new sql.ConnectionPool(config).connect().then(pool => {  return pool.request().query("SELECT * FROM MyTable")  }).then(result => {    let rows = result.recordset    res.setHeader('Access-Control-Allow-Origin', '*')    res.status(200).json(rows);    sql.close();  }).catch(err => {    res.status(500).send({ message: `${err}`})    sql.close();  });

From the documentation, close method should be used on the connection, and not on the required module,

So should be used like

var connection = new sql.Connection({user: '...',password: '...',server: 'localhost',database: '...'});connection.close().

Also couple of suggestions,
1. putting res.send in a loop isn't a good idea, You could reply back the entire recordsets or do operations over it, store the resultant in a variable and send that back.
2. Try using promises, instead of callbacks, it would make the flow neater

You must use ConnectionPool.

Next function returns a recordset with my query results.

async function execute2(query) {    return new Promise((resolve, reject) => {        new sql.ConnectionPool(dbConfig).connect().then(pool => {            return pool.request().query(query)        }).then(result => {            resolve(result.recordset);            sql.close();        }).catch(err => {            reject(err)            sql.close();        });    });}