cron-emitter

This is an event emitter that uses crontab instructions to register events to be emitted at regular intervals, or at certain specific times in the future. This module uses cron-parser to parse the crontab instructions. See also node-cron for a similar project with a more traditional node.js callback approach.

The main difference between this Library and node-cron is that this library aims to implement the event listener pattern, to achieve capabilities found in the Observer Pattern and the PUB/SUB Pattern

This enables you to write highly decoupled, functional and reactive code with lower complexity per function than callback driven code. You can delegate responsibility for dealing with events to as many different subsystems as we want, all listening to the same event notifications.

Table of contents

• Install
• Crontab syntax
• Usage
  • Creating an Object
  • Adding an event
  • Options
• API
  • constructor
  • add
  • remove
  • getEventList
• Example

Install

$ npm install --save cron-emitter

Crontab syntax

See cron-parser for a simple introduction to the crontab syntax. If you're on a Linux or OS X computer type

$ man crontab

Usage

Creating an object

const CronEmitter = require('cron-emitter');
const emitter     = new CronEmitter();

Adding an event

emitter.add('0 */30 * * * *', 'every_thirty_minutes');

Options

CronEmitter exposes the same options as cron-parser to provide a start date and an end date for when events should be emitted:

emitter.add('0 0 */2 * * *', 'every_two_hours', {
  currentDate: new Date(),
  endDate: '2017-05-31'
});

API

new CronEmitter()

Return a new CronEmitter object

cronEmitter.add(crontab, name, options) ⇒ CronEmitter

Adds a new event to the list of events to be emitted. CronEmitter exposes the same options as cron-parser to provide a start date and an end date for when events should be emitted:

• Kind: instance method of CronEmitter
• Returns: CronEmitter - A reference to self.
• See: https://www.npmjs.com/package/cron-parser

Param	Type	Description
crontab	string	the crontab declaration
name	string	the name of the event you want to emit.
options	object	an object with options to cron-parser

cronEmitter.remove(name) ⇒ CronEmitter

Remove an event from the list of events

• Kind: instance method of CronEmitter
• Returns: CronEmitter - self when successful
• Throws:
  • TypeError

Param	Type	Description
name	string	of the event to remove.

cronEmitter.getEventList() ⇒ object

Returns object with all the registered emitters

• Kind: instance method of CronEmitter
• Returns: object - the list of events.

Example

const CronEmitter = require('../lib/cron-emitter');
const emitter     = new CronEmitter();
 
const now       = () => (new Date()).toJSON();
const increment = (counter = 0) => (counter + 1);
 
emitter.add('*/3  * * * * *', 'every_three_seconds');
emitter.add('*/10 * * * * *', 'every_ten_seconds');
emitter.add('0 * * * * *',    'every_minute');
emitter.add('0 */5 * * * *',  'every_five_minutes');
emitter.add('0 */30 * * * *', 'every_thirty_minutes');
emitter.add('* * * * * *',    'every_second_stop', {
  endDate: new Date(Date.now() + 10500)
});
 
console.log(now(), "==> Done setting up events");
 
emitter.on('ended', (name) => {
  console.log(now(), `==> ENDED: event series "${name}" has ended.`);
  switch (name) {
  case 'every_second_stop':
    emitter.add(
      '*/2 * * * * *', 'every_two_seconds',
      {endDate: new Date(Date.now() + 10500)}
      );
    break;
  }
});
 
emitter.on('every_three_seconds', () => {
  console.log(now(), "==> EVENT: Got every three seconds event.");
});
 
emitter.on('every_ten_seconds', () => {
  console.log(now(), "==> EVENT: Got ten seconds event.");
});
 
emitter.on('every_minute', () => {
  console.log(now(), "==> EVENT: A minute has passed");
});
 
emitter.on('every_five_minutes', () => {
  console.log(now(), "==> EVENT: Five minutes has passed");
});
 
emitter.on('every_thirty_minutes', () => {
  console.log(now(), "==> EVENT: Thirty minutes has passed");
});
 
const increment = (counter = 0) => (counter + 1);
let counter = 0;
 
emitter.on("every_second_stop", () => {
  counter = increment(counter);
  console.log(now(), "==> EVENT: got every second event: ", counter);
});
 
emitter.on('every_two_seconds', () => {
  console.log(now(), "==> EVENT: got every two seconds event.");
});