cron

What is cron?

The cron npm package is used for scheduling tasks to be executed at specific times or intervals. It is inspired by the Unix cron scheduler and allows for the use of cron syntax to schedule tasks in a Node.js environment. This package is useful for setting up jobs like backups, sending emails, or cleaning up databases at regular intervals.

What are cron's main functionalities?

Basic Cron Job

This feature allows you to create a basic cron job that runs at a specified interval. In the provided code sample, a new CronJob is created that logs a message to the console every second.

"const CronJob = require('cron').CronJob;\nconst job = new CronJob('* * * * * *', function() {\n  console.log('You will see this message every second');\n}, null, true, 'America/Los_Angeles');\njob.start();"

Time Zone Support

This feature demonstrates the ability to specify a time zone for the cron job. The code sample schedules a job to run at 11:30 AM, according to the 'America/New_York' time zone, from Monday to Friday.

"const CronJob = require('cron').CronJob;\nconst job = new CronJob('00 30 11 * * 1-5', function() {\n  console.log('This runs at 11:30 AM (server time) every Monday through Friday.');\n}, null, true, 'America/New_York');\njob.start();"

Dynamic Job Scheduling

This feature allows for dynamic scheduling of jobs. The schedule can be updated or changed based on certain conditions or inputs. In the example, the 'dynamicSchedule' variable can be updated to change the job's schedule.

"const CronJob = require('cron').CronJob;\nlet dynamicSchedule = '00 30 11 * * 1-5'; // This can be dynamically changed\nconst job = new CronJob(dynamicSchedule, function() {\n  console.log('This job's schedule can be dynamically changed.');\n}, null, true, 'America/Los_Angeles');\njob.start();"

Other packages similar to cron

node-schedule

node-schedule is a flexible cron-like and not-cron-like job scheduler for Node.js. It allows for more complex scheduling and includes features like job cancellation. It is a good alternative to cron when more flexibility is required.

agenda

Agenda is a job scheduling library for Node.js that uses MongoDB for job storage. It offers more robust job management features compared to cron, such as persistence, job prioritization, and repeating jobs. It's a good choice when jobs need to be managed across server restarts or in distributed systems.

bull

Bull is a Redis-based queue system for Node.js. It is not a direct alternative to cron but can be used for scheduling through delayed jobs. It offers advanced features like job prioritization, concurrency control, and job events. Bull is suitable for applications requiring high reliability and real-time processing.

README

cron is a robust tool for running jobs (functions or commands) on schedules defined using the cron syntax.
Perfect for tasks like data backups, notifications, and many more!

Cron for Node.js

🌟 Features

• execute a function whenever your scheduled job triggers
• execute a job external to the javascript process (like a system command) using child_process
• use a Date or Luxon DateTime object instead of cron syntax as the trigger for your callback
• use an additional slot for seconds (leaving it off will default to 0 and match the Unix behavior)

🚀 Installation

npm install cron

Table of Contents

1. Features
2. Installation
3. Migrating from v2 to v3
4. Basic Usage
5. Cron Patterns
   • Cron Syntax Overview
   • Supported Ranges
6. Gotchas
7. API
   • Standalone Functions
   • CronJob Class
   • CronTime Class
8. Community
   • Join the Community
9. Contributing
   • General Contribution
   • Submitting Bugs/Issues
10. Acknowledgements
11. License

🔄 Migrating from v2 to v3

With the introduction of TypeScript in version 3 and alignment with UNIX cron patterns, a few changes have been made:

Migrating from v2 to v3

Month & day-of-week indexing changes

• Month Indexing: Changed from 0-11 to 1-12. So you need to increment all numeric months by 1.

• Day-of-Week Indexing: Support added for 7 as Sunday.

Adjustments in CronJob

• The constructor no longer accepts an object as its first and only params. Use CronJob.from(argsObject) instead.
• Callbacks are now called in the order they were registered.
• nextDates(count?: number) now always returns an array (empty if no argument is provided). Use nextDate() instead for a single date.

Removed methods

• removed job() method in favor of new CronJob(...args) / CronJob.from(argsObject)

• removed time() method in favor of new CronTime()

🛠 Basic Usage

import { CronJob } from 'cron';

const job = new CronJob(
	'* * * * * *', // cronTime
	function () {
		console.log('You will see this message every second');
	}, // onTick
	null, // onComplete
	true, // start
	'America/Los_Angeles' // timeZone
);
// job.start() is optional here because of the fourth parameter set to true.

// equivalent job using the "from" static method, providing parameters as an object
const job = CronJob.from({
	cronTime: '* * * * * *',
	onTick: function () {
		console.log('You will see this message every second');
	},
	start: true,
	timeZone: 'America/Los_Angeles'
});

Note: In the first example above, the fourth parameter to CronJob() starts the job automatically. If not provided or set to falsy, you must explicitly start the job using job.start().

For more advanced examples, check the examples directory.

Cron Patterns

Cron patterns are the backbone of this library. Familiarize yourself with the syntax:

- `*` Asterisks: Any value
- `1-3,5` Ranges: Ranges and individual values
- `*/2` Steps: Every two units

Detailed patterns and explanations are available at crontab.org. The examples in the link have five fields, and 1 minute as the finest granularity, but our cron scheduling supports an enhanced format with six fields, allowing for second-level precision. Tools like crontab.guru can help in constructing patterns but remember to account for the seconds field.

Supported Ranges

Here's a quick reference to the UNIX Cron format this library uses, plus an added second field:

field          allowed values
-----          --------------
second         0-59
minute         0-59
hour           0-23
day of month   1-31
month          1-12 (or names, see below)
day of week    0-7 (0 or 7 is Sunday, or use names)

Names can also be used for the 'month' and 'day of week' fields. Use the first three letters of the particular day or month (case does not matter). Ranges and lists of names are allowed.
Examples: "mon,wed,fri", "jan-mar".

Gotchas

• Both JS Date and Luxon DateTime objects don't guarantee millisecond precision due to computation delays. This module excludes millisecond precision for standard cron syntax but allows execution date specification through JS Date or Luxon DateTime objects. However, specifying a precise future execution time, such as adding a millisecond to the current time, may not always work due to these computation delays. It's observed that delays less than 4-5 ms might lead to inconsistencies. While we could limit all date granularity to seconds, we've chosen to allow greater precision but advise users of potential issues.

• Using arrow functions for onTick binds them to the parent's this context. As a result, they won't have access to the cronjob's this context. You can read a little more in issue #47 (comment).

API

Standalone Functions

• sendAt: Indicates when a CronTime will execute (returns a Luxon DateTime object).

  import * as cron from 'cron';
  
  const dt = cron.sendAt('0 0 * * *');
  console.log(`The job would run at: ${dt.toISO()}`);
  

• timeout: Indicates the number of milliseconds in the future at which a CronTime will execute (returns a number).

  import * as cron from 'cron';
  
  const timeout = cron.timeout('0 0 * * *');
  console.log(`The job would run in ${timeout}ms`);
  

CronJob Class

Constructor

constructor(cronTime, onTick, onComplete, start, timeZone, context, runOnInit, utcOffset, unrefTimeout):

• cronTime: [REQUIRED] - The time to fire off your job. Can be cron syntax, a JS Date object or a Luxon DateTime object.

• onTick: [REQUIRED] - Function to execute at the specified time. If an onComplete callback was provided, onTick will receive it as an argument.

• onComplete: [OPTIONAL] - Invoked when the job is halted with job.stop(). It might also be triggered by onTick post its run.

• start: [OPTIONAL] - Determines if the job should commence before constructor exit. Default is false.

• timeZone: [OPTIONAL] - Sets the execution time zone. Default is local time. Check valid formats in the Luxon documentation.

• context: [OPTIONAL] - Execution context for the onTick method.

• runOnInit: [OPTIONAL] - Instantly triggers the onTick function post initialization. Default is false.

• utcOffset: [OPTIONAL] - Specifies time zone offset in minutes. Cannot co-exist with timeZone.

• unrefTimeout: [OPTIONAL] - Useful for controlling event loop behavior. More details here.

• waitForCompletion: [OPTIONAL] - If true, no additional instances of the onTick callback function will run until the current onTick callback has completed. Any new scheduled executions that occur while the current callback is running will be skipped entirely. Default is false.

Methods

• from (static): Create a new CronJob object providing arguments as an object. See argument names and descriptions above.

• start: Initiates the job.

• stop: Halts the job.

• setTime: Modifies the time for the CronJob. Parameter must be a CronTime.

• lastDate: Provides the last execution date.

• nextDate: Indicates the subsequent date that will activate an onTick.

• nextDates(count): Supplies an array of upcoming dates that will initiate an onTick.

• fireOnTick: Allows modification of the onTick calling behavior.

• addCallback: Permits addition of onTick callbacks.

Properties

• isCallbackRunning: [READ-ONLY] Indicates if a callback is currently executing.

  const job = new CronJob('* * * * * *', async () => {
  	console.log(job.isCallbackRunning); // true during callback execution
  	await someAsyncTask();
  	console.log(job.isCallbackRunning); // still true until callback completes
  });
  
  console.log(job.isCallbackRunning); // false
  job.start();
  console.log(job.running); // true (schedule is active)
  console.log(job.isCallbackRunning); // false (no callback executing)
  

CronTime Class

Constructor

constructor(time, zone, utcOffset):

• time: [REQUIRED] - The time to initiate your job. Accepts cron syntax or a JS Date object.

• zone: [OPTIONAL] - Equivalent to timeZone from CronJob parameters.

• utcOffset: [OPTIONAL] - Analogous to utcOffset from CronJob parameters.

🤝 Community

Join the Discord server! Here you can discuss issues and get help in a more casual forum than GitHub.

🌍 Contributing

This project is looking for help! If you're interested in helping with the project, please take a look at our contributing documentation.

🐛 Submitting Bugs/Issues

Please have a look at our contributing documentation, it contains all the information you need to know before submitting an issue.

🙏 Acknowledgements

This is a community effort project. In the truest sense, this project started as an open source project from cron.js and grew into something else. Other people have contributed code, time, and oversight to the project. At this point there are too many to name here so we'll just say thanks.

Special thanks to Hiroki Horiuchi, Lundarl Gholoi and koooge for their work on the DefinitelyTyped typings before they were imported in v2.4.0.

License

MIT