A sample BLE peripheral service and characteristic for use with Node.js and bleno.

characteristic.js

/*  characteristic.js 
 *  A simple custom BLE peripheral characteristic for use with Node.js and bleno.
 *  This characteristic supports read, write, and notify properties.
 *  Julian Hays - 10/14/19
 */

var util = require('util');
var bleno = require('bleno-mac'); //or 'bleno-mac' if you are using that
var BlenoCharacteristic = bleno.Characteristic;

var CustomCharacteristic = function() {
	CustomCharacteristic.super_.call(this, {
		uuid: 'fd758b93-0bfa-4c52-8af0-85845a74a606',
		properties: ['read', 'write', 'notify']
	});

	this._value = new Buffer(0);
	this._updateValueCallback = null;
};

util.inherits(CustomCharacteristic, BlenoCharacteristic);
module.exports = CustomCharacteristic;

CustomCharacteristic.prototype.onReadRequest = function (offset, callback) {
	console.log('CustomCharacteristic onReadRequest');
	var data = new Buffer(1);
	data.writeUInt8(42, 0);
	callback(this.RESULT_SUCCESS, data);
};

CustomCharacteristic.prototype.onWriteRequest = function(data, offset, withoutResponse, callback) {
	this._value = data;
	console.log('CustomCharacteristic - onWriteRequest: value = ' + this._value.toString('hex'));
	callback(this.RESULT_SUCCESS);
};

var isSubscribed = false
var notifyInterval = 5 //seconds

function delayedNotification(callback) {
	setTimeout(function() { 
		if (isSubscribed) {
			var data = Buffer(3);
			var now = new Date();
			data.writeUInt8(now.getHours(), 0);
			data.writeUInt8(now.getMinutes(), 1);
			data.writeUInt8(now.getSeconds(), 2);
			callback(data);
			delayedNotification(callback);
		}
	}, notifyInterval * 1000);
}

CustomCharacteristic.prototype.onSubscribe = function(maxValueSize, updateValueCallback) {
	console.log('CustomCharacteristic - onSubscribe');
	isSubscribed = true;
	delayedNotification(updateValueCallback);
	this._updateValueCallback = updateValueCallback;
};

CustomCharacteristic.prototype.onUnsubscribe = function() {
	console.log('CustomCharacteristic - onUnsubscribe');
	isSubscribed = false;
	this._updateValueCallback = null;
};

service.js

/*  service.js 
 *  A simple custom BLE peripheral service for use with Node.js and bleno.
 *  Julian Hays - 10/14/19
 */

var bleno = require('bleno-mac') ; 
var BlenoPrimaryService = bleno.PrimaryService;

bleno.on('stateChange', function(state) {
console.log('on -> stateChange: ' + state);
	if (state === 'poweredOn') {
		console.log("request startAdvertising");
		bleno.startAdvertising('CustomService', ['27cf08c1-076a-41af-becd-02ed6f6109b9']);  
	} else {
		console.log("request stopAdvertising");
		bleno.stopAdvertising(); 
	}
});

var CustomCharacteristic = require('./characteristic');

bleno.on('advertisingStart', function(error) {
	console.log('on -> advertisingStart: ' + (error ? 'error ' + error : 'success'));

	if (!error) {
		bleno.setServices([
			new BlenoPrimaryService({
				uuid: '27cf08c1-076a-41af-becd-02ed6f6109b9',
				characteristics: [
					new CustomCharacteristic()
				]
			})
		]);
	}
});

Thanks for sharing the code and guide link.
Your code and step-by-step guide is very well explained.

A step-by-step tutorial on this code is available on the Punch Through blog:
https://punchthrough.com/how-to-use-node-js-to-speed-up-ble-app-development/

How can we show multiple characteristics? Adding new is not working (see below code example)

characteristics: [ new CustomCharacteristic(), new NewCustomCharacteristic(), // Add new here ]

help me to add multiple services and/or characteristics.