// mongoose 4.3.x
var mongoose = require('mongoose');
 
/* 
 * Mongoose by default sets the auto_reconnect option to true.
 * We recommend setting socket options at both the server and replica set level.
 * We recommend a 30 second connection timeout because it allows for 
 * plenty of time in most operating environments.
 */
var options = { server: { socketOptions: { keepAlive: 300000, connectTimeoutMS: 30000 } }, 
                replset: { socketOptions: { keepAlive: 300000, connectTimeoutMS : 30000 } } };       
 
var mongodbUri = 'mongodb://user:pass@host:port/db';
 
mongoose.connect(mongodbUri, options);
var conn = mongoose.connection;             
 
conn.on('error', console.error.bind(console, 'connection error:'));  
 
conn.once('open', function() {
  // Wait for the database connection to establish, then start the app.                         
});

...