mLab recommended mongoose connection options. More supported connections for the underlying Node Native driver can be found here: http://mongodb.github.io/node-mongodb-native/

mongoose-connection-options.js

// 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.                         
});

...

connecting your app with mlab database

const mongoose = require('mongoose');
var mongodbUri ='mongodb://@ds249992.mlab.com:49992/databasename';
mongoose.connect(mongodbUri, {
  useNewUrlParser: true,
  auth: {
    user: 'UserName',
    password: 'Password'
  }
})
var conn = mongoose.connection;    
conn.on('error', console.error.bind(console, 'connection error:'));  
 
conn.once('open', () =>{
 console.log('connected to adatabase')                       
});

@taourate tx code is fine and working

you saved me too :)

i connected my mlab acc ,but i'm not able to access the collection in the database