use a single mssql connection pool across several files using a promise

db.js

const sql = require('mssql')
const config = {}

const pool = new sql.ConnectionPool(config)
  .connect()
  .then(connPool => {
    console.log('Connected to MSSQL')
    return connPool
  })
  .catch(err => console.log('Database Connection Failed! Bad Config: ', err))

module.exports = {
  sql, pool
}

query.js

const { pool, sql } = require('./db')

return pool.then(conn => {
    const ps = new sql.PreparedStatement(conn)
    ps.input('xxxx', sql.VarChar)

    return ps.prepare(`SELECT * from table where xxxx = @xxxx`)
      .then(data => ps.execute({ xxxx: 'xxxx' }))
  })