cron-parser

A JavaScript library for parsing and manipulating cron expressions. Features timezone support, DST handling, and iterator capabilities.

API documentation

Requirements

• Node.js >= 18
• TypeScript >= 5

Installation

npm install cron-parser

Cron Format

*    *    *    *    *    *
┬    ┬    ┬    ┬    ┬    ┬
│    │    │    │    │    │
│    │    │    │    │    └─ day of week (0-7, 1L-7L) (0 or 7 is Sun)
│    │    │    │    └────── month (1-12, JAN-DEC)
│    │    │    └─────────── day of month (1-31, L)
│    │    └──────────────── hour (0-23)
│    └───────────────────── minute (0-59)
└────────────────────────── second (0-59, optional)

Special Characters

Character	Description	Example
*	Any value	* * * * * (every minute)
?	Any value (alias for *)	? * * * * (every minute)
,	Value list separator	1,2,3 * * * * (1st, 2nd, and 3rd minute)
-	Range of values	1-5 * * * * (every minute from 1 through 5)
/	Step values	*/5 * * * * (every 5th minute)
L	Last day of month/week	0 0 L * * (midnight on last day of month)
#	Nth day of month	0 0 * * 1#1 (first Monday of month)

Predefined Expressions

Expression	Description	Equivalent
@yearly	Once a year at midnight of January 1	0 0 0 1 1 *
@monthly	Once a month at midnight of first day	0 0 0 1 * *
@weekly	Once a week at midnight on Sunday	0 0 0 * * 0
@daily	Once a day at midnight	0 0 0 * * *
@hourly	Once an hour at the beginning of the hour	0 0 * * * *
@minutely	Once a minute	0 * * * * *
@secondly	Once a second	* * * * * *
@weekdays	Every weekday at midnight	0 0 0 * * 1-5
@weekends	Every weekend at midnight	0 0 0 * * 0,6

Field Values

Field	Values	Special Characters	Aliases
second	0-59	* ? , - /
minute	0-59	* ? , - /
hour	0-23	* ? , - /
day of month	1-31	* ? , - / L
month	1-12	* ? , - /	JAN-DEC
day of week	0-7	* ? , - / L #	SUN-SAT (0 or 7 is Sunday)

Options

Option	Type	Description
currentDate	Date | string | number	Current date. Defaults to current local time in UTC
endDate	Date | string | number	End date of iteration range. Sets iteration range end point
startDate	Date | string | number	Start date of iteration range. Set iteration range start point
tz	string	Timezone (e.g., 'Europe/London')
strict	boolean	Enable strict mode validation

When using string dates, the following formats are supported:

• ISO8601
• HTTP and RFC2822
• SQL

Basic Usage

Expression Parsing

import { CronExpressionParser } from 'cron-parser';

try {
  const interval = CronExpressionParser.parse('*/2 * * * *');

  // Get next date
  console.log('Next:', interval.next().toString());
  // Get next 3 dates
  console.log(
    'Next 3:',
    interval.take(3).map((date) => date.toString()),
  );

  // Get previous date
  console.log('Previous:', interval.prev().toString());
} catch (err) {
  console.log('Error:', err.message);
}

With Options

import { CronExpressionParser } from 'cron-parser';

const options = {
  currentDate: '2023-01-01T00:00:00Z',
  endDate: '2024-01-01T00:00:00Z',
  tz: 'Europe/London',
};

try {
  const interval = CronExpressionParser.parse('0 0 * * *', options);
  console.log('Next:', interval.next().toString());
} catch (err) {
  console.log('Error:', err.message);
}

Crontab File Operations

For working with crontab files, use the CronFileParser:

import { CronFileParser } from 'cron-parser';

// Async file parsing
try {
  const result = await CronFileParser.parseFile('/path/to/crontab');
  console.log('Variables:', result.variables);
  console.log('Expressions:', result.expressions);
  console.log('Errors:', result.errors);
} catch (err) {
  console.log('Error:', err.message);
}

// Sync file parsing
try {
  const result = CronFileParser.parseFileSync('/path/to/crontab');
  console.log('Variables:', result.variables);
  console.log('Expressions:', result.expressions);
  console.log('Errors:', result.errors);
} catch (err) {
  console.log('Error:', err.message);
}

Advanced Features

Strict Mode

In several implementations of CRON, it's ambiguous to specify both the Day Of Month and Day Of Week parameters simultaneously, as it's unclear which one should take precedence. Despite this ambiguity, this library allows both parameters to be set by default, although the resultant behavior might not align with your expectations.

To resolve this ambiguity, you can activate the strict mode of the library. In strict mode, the library prevents the simultaneous setting of both Day Of Month and Day Of Week, effectively serving as a validation method for user inputs.

import { CronExpressionParser } from 'cron-parser';

// Specifies a schedule that occurs at 12:00 on every day-of-month from 1 through 31 and on Monday
const options = {
  currentDate: new Date('Mon, 12 Sep 2022 14:00:00'),
  strict: true,
};

try {
  // This will throw an error in strict mode
  CronExpressionParser.parse('0 0 12 1-31 * 1', options);
} catch (err) {
  console.log('Error:', err.message);
  // Error: Cannot use both dayOfMonth and dayOfWeek together in strict mode!
}

Last Day of Month/Week Support

The library supports parsing the range 0L - 7L in the weekday position of the cron expression, where the L means "last occurrence of this weekday for the month in progress".

For example, the following expression will run on the last Monday of the month at midnight:

import { CronExpressionParser } from 'cron-parser';

// Last Monday of every month at midnight
const lastMonday = CronExpressionParser.parse('0 0 0 * * 1L');

// You can also combine L expressions with other weekday expressions
// This will run every Monday and the last Wednesday of the month
const mixedWeekdays = CronExpressionParser.parse('0 0 0 * * 1,3L');

// Last day of every month
const lastDay = CronExpressionParser.parse('0 0 L * *');

Using Iterator

import { CronExpressionParser } from 'cron-parser';

const interval = CronExpressionParser.parse('0 */2 * * *');

// Using for...of
for (const date of interval) {
  console.log('Iterator value:', date.toString());
  if (someCondition) break;
}

// Using take() for a specific number of iterations
const nextFiveDates = interval.take(5);
console.log(
  'Next 5 dates:',
  nextFiveDates.map((date) => date.toString()),
);

Timezone Support

The library provides robust timezone support using Luxon, handling DST transitions correctly:

import { CronExpressionParser } from 'cron-parser';

const options = {
  currentDate: '2023-03-26T01:00:00',
  tz: 'Europe/London',
};

const interval = CronExpressionParser.parse('0 * * * *', options);

// Will correctly handle DST transition
console.log('Next dates during DST transition:');
console.log(interval.next().toString());
console.log(interval.next().toString());
console.log(interval.next().toString());

Field Manipulation

You can modify cron fields programmatically using CronFieldCollection.from and construct a new expression:

import { CronExpressionParser, CronFieldCollection, CronHour, CronMinute } from 'cron-parser';

// Parse original expression
const interval = CronExpressionParser.parse('0 7 * * 1-5');

// Create new collection with modified fields using raw values
const modified = CronFieldCollection.from(interval.fields, {
  hour: [8],
  minute: [30],
  dayOfWeek: [1, 3, 5],
});

console.log(modified.stringify()); // "30 8 * * 1,3,5"

// You can also use CronField instances
const modified2 = CronFieldCollection.from(interval.fields, {
  hour: new CronHour([15]),
  minute: new CronMinute([30]),
});

console.log(modified2.stringify()); // "30 15 * * 1-5"

The CronFieldCollection.from method accepts either CronField instances or raw values that would be valid for creating new CronField instances. This is particularly useful when you need to modify only specific fields while keeping others unchanged.

License

MIT