[MongoDb][Node.js] A simple demo of write concern settings when inserting documents on a replica set

write_concerns.js

/**
 * Simple code to demonstrate write concerns setting when inserting documents into replica set.
 * Run this command to install required dependency, in this source code's directory: npm install mongodb
 */

var MongoClient = require('mongodb').MongoClient;
var assert = require('assert');

// Notice the url query string in the below connection string to replica set. We define write concern = 1 as the default write concern setting.
// w=1 means, the driver would returns success once a document has been successfully inserted into the Primary Node. 
var connection_string = "mongodb://KURONO:27018, KURONO:27019, KURONO:27020/demo_replica_set?w=1";

MongoClient.connect(connection_string, function(err, db){
	if (err) throw err;

	console.log("[INFO] - Connection to '"+connection_string+"' is established...");

	var test_docs = db.collection('test_docs');
	// Create a document & insert it into the database
	var test_doc = {
		'value': Math.random()
	};

	test_docs.insert(test_doc, function(err, inserted){
		if (err) throw err;

		// Retrieve the created document 
		assert.ok(inserted != null);

		console.log("[INFO] - Successfully inserted "+ JSON.stringify(inserted)+" document into the database using write concerns setting 'w=1'.");

		var second_test_doc = {'value': Math.random()};
		// w=2 means, the driver would returns success once a document has been successfully inserted into the Primary Node & also being synched to the Secondary node. 
		var write_concern = {'w':2};

		//  Insert another document with different write concerns setting (e.g. w=2).  		
		test_docs.insert(second_test_doc, function(err, inserted){
			if (err) throw err;

			// Retrieve the created document 
			assert.ok(inserted != null);

			console.log("[INFO] - Successfully inserted "+ JSON.stringify(inserted)+" document into the database using write concerns setting: "+JSON.stringify(write_concern));
			console.log("[INFO] - Closing database connection ...");
			return db.close();
		});
	});
});

this is misleading.
defining a local var write_concern has nothing to do with your insert.

You just defined a local variable.