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

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 have to create a poolConnection try 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();

  });