How to get mongodb version from mongoose

You can use the native mongo driver's Admin#buildInfo method for that via your Mongoose connection:

var mongoose = require('mongoose');

mongoose.connect('mongodb://localhost/test', function(err){
  var admin = new mongoose.mongo.Admin(mongoose.connection.db);
  admin.buildInfo(function (err, info) {
     console.log(info.version);
  });
});

You can query the buildInfo directly from your Mongoose connection.

var mongoose = require('mongoose');

mongoose.connect('mongodb://localhost/test', function(err) {
    mongoose.db.command({ buildInfo: 1 }, function (err, info) {
        console.log(info.version);
    });
});

https://docs.mongodb.com/manual/reference/command/buildInfo/#dbcmd.buildInfo