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

node-cron

Cron is a tool that allows you to execute something on a schedule. This is typically done using the cron syntax. We allow you to execute a function whenever your scheduled job triggers. We also allow you to execute a job external to the javascript process using child_process. Additionally, this library goes beyond the basic cron syntax and allows you to supply a Date object. This will be used as the trigger for your callback. Cron syntax is still an acceptable CronTime format. Although the Cron patterns supported here extend on the standard Unix format to support seconds digits, leaving it off will default to 0 and match the Unix behavior.

Installation

npm install cron

If You Are Submitting Bugs/Issues

Because we can't magically know what you are doing to expose an issue, it is best if you provide a snippet of code. This snippet need not include your secret sauce, but it must replicate the issue you are describing. The issues that get closed without resolution tend to be the ones without code examples. Thanks.

Versions and Backwards compatibility breaks:

As goes with semver, breaking backwards compatibility should be explicit in the versioning of your library. As such, we'll upgrade the version of this module in accordance with breaking changes (I'm not always great about doing it this way so if you notice that there are breaking changes that haven't been bumped appropriately please let me know).

Usage (basic cron usage):

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

Note - You need to explicitly start a job in order to make it run. This gives a little more control over running your jobs.

There are more examples available in this repository at: /examples

Available Cron patterns:

Asterisk. E.g. *
Ranges. E.g. 1-3,5
Steps. E.g. */2

Read up on cron patterns here. Note the examples in the link have five fields, and 1 minute as the finest granularity, but this library has six fields, with 1 second as the finest granularity.

There are tools that help when constructing your cronjobs. You might find something like https://crontab.guru/ or https://cronjob.xyz/ helpful. But, note that these don't necessarily accept the exact same syntax as this library, for instance, it doesn't accept the seconds field, so keep that in mind.

Cron Ranges

When specifying your cron values you'll need to make sure that your values fall within the ranges. For instance, some cron's use a 0-7 range for the day of week where both 0 and 7 represent Sunday. We do not. And that is an optimisation.

• Seconds: 0-59
• Minutes: 0-59
• Hours: 0-23
• Day of Month: 1-31
• Months: 0-11 (Jan-Dec)
• Day of Week: 0-6 (Sun-Sat)

Gotchas

• Millisecond level granularity in JS or moment date objects. Because computers take time to do things, there may be some delay in execution. This should be on the order of milliseconds. This module doesn't allow MS level granularity for the regular cron syntax, but does allow you to specify a real date of execution in either a javascript date object or a moment object. When this happens you may find that you aren't able to execute a job that should run in the future like with new Date().setMilliseconds(new Date().getMilliseconds() + 1). This is due to those cycles of execution above. This wont be the same for everyone because of compute speed. When I tried it locally I saw that somewhere around the 4-5 ms mark was where I got consistent ticks using real dates, but anything less than that would result in an exception. This could be really confusing. We could restrict the granularity for all dates to seconds, but felt that it wasn't a huge problem so long as you were made aware. If this becomes more of an issue, We can revisit it.
• Arrow Functions for onTick Arrow functions get their this context from their parent scope. Thus, if you use them, you will not get the this context of the cronjob. You can read a little more in this ticket GH-40

API

Parameter Based

• job - shortcut to new cron.CronJob().
• time - shortcut to new cron.CronTime().
• sendAt - tells you when a CronTime will be run.
• timeout - tells you when the next timeout is.
• CronJob
  • constructor(cronTime, onTick, onComplete, start, timezone, context, runOnInit, unrefTimeout) - Of note, the first parameter here can be a JSON object that has the below names and associated types (see examples above).
    • cronTime - [REQUIRED] - The time to fire off your job. This can be in the form of cron syntax or a JS Date object.
    • onTick - [REQUIRED] - The function to fire at the specified time. If an onComplete callback was provided, onTick will receive it as an argument. onTick may call onComplete when it has finished its work.
    • onComplete - [OPTIONAL] - A function that will fire when the job is stopped with job.stop(), and may also be called by onTick at the end of each run.
    • start - [OPTIONAL] - Specifies whether to start the job just before exiting the constructor. By default this is set to false. If left at default you will need to call job.start() in order to start the job (assuming job is the variable you set the cronjob to). This does not immediately fire your onTick function, it just gives you more control over the behavior of your jobs.
    • timeZone - [OPTIONAL] - Specify the timezone for the execution. This will modify the actual time relative to your timezone. If the timezone is invalid, an error is thrown. You can check all timezones available at Moment Timezone Website. Probably don't use both. timeZone and utcOffset together or weird things may happen.
    • context - [OPTIONAL] - The context within which to execute the onTick method. This defaults to the cronjob itself allowing you to call this.stop(). However, if you change this you'll have access to the functions and values within your context object.
    • runOnInit - [OPTIONAL] - This will immediately fire your onTick function as soon as the requisite initialization has happened. This option is set to false by default for backwards compatibility.
    • utcOffset - [OPTIONAL] - This allows you to specify the offset of your timezone rather than using the timeZone param. Probably don't use both timeZone and utcOffset together or weird things may happen.
    • unrefTimeout - [OPTIONAL] - If you have code that keeps the event loop running and want to stop the node process when that finishes regardless of the state of your cronjob, you can do so making use of this parameter. This is off by default and cron will run as if it needs to control the event loop. For more information take a look at timers#timers_timeout_unref from the NodeJS docs.
  • start - Runs your job.
  • stop - Stops your job.
  • setTime - Stops and changes the time for the CronJob. Param must be a CronTime.
  • lastDate - Tells you the last execution date.
  • nextDates - Provides an array of the next set of dates that will trigger an onTick.
  • fireOnTick - Allows you to override the onTick calling behavior. This matters so only do this if you have a really good reason to do so.
  • addCallback - Allows you to add onTick callbacks.
• CronTime
  • constructor(time)
    • time - [REQUIRED] - The time to fire off your job. This can be in the form of cron syntax or a JS Date object.

Contributions

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 I'll just say thanks.

License

MIT

Changelog

v1.8.2 (2020-01-24)

• Fix syntax parsing bug