Create mssql connection pool in node.js application

You would find that information in the page you're referring to yourself. But it's worth noting that the documenting is not explicit about how a Connection pool is created.

var connection = new sql.Connection(config);

The variable connection here is actually a connection pool that still needs to connect...

var connection = new sql.Connection(config, function(err) {
  // ... error checks

  // Query
  var request = new sql.Request(connection);
   // or: var request = connection.request();
  request.query('select 1 as number', function(err, recordset) {
    // ... error checks
    console.dir(recordset);
  });
});

Or using promises by omitting a callback argument

var connection = new sql.Connection(config);
//connection is actually a connection pool
connection.connect().then(function() {

    var request = new sql.Request(connection);
    request.query('select * from mytable').then(function(recordset) {
        // ...
    }).catch(function(err) {
        // ...
    });
}).catch(function(err) {
    // ...
});

After you create a single instance of Connection, let all your requests use it like in the example above as opposed to instantiating a new Connection object for each request.

If your asking in relation to a Express 4 web application, see this question and answers.