Scheduled tasks (cron) in Deno

Introduction

Scheduled tasks let your program do things automatically at set times. This helps you run jobs like backups or reminders without needing to start them manually.

Send daily email reports at 8 AM.

Clean up temporary files every hour.

Run a database backup every night at midnight.

Check for updates every 10 minutes.

Send reminders every Monday morning.

Syntax

Deno

import { Cron } from "https://deno.land/x/croner@6.0.0/mod.ts";

const cron = new Cron("* * * * *", () => {
  // code to run every minute
});

cron.start();

The string "* * * * *" is a cron expression that sets when the task runs.

Use cron.start() to begin the scheduled task.

Examples

This runs the task every day at 8:00 AM.

Deno

const cron = new Cron("0 8 * * *", () => {
  console.log("Good morning! Time to start work.");
});
cron.start();

This runs the task every 15 minutes.

Deno

const cron = new Cron("*/15 * * * *", () => {
  console.log("Checking status every 15 minutes.");
});
cron.start();

This runs the task every Sunday at midnight.

Deno

const cron = new Cron("0 0 * * 0", () => {
  console.log("Weekly report generated.");
});
cron.start();

Sample Program

This program prints a message every minute. After 3 minutes, it stops the cron job and prints a stop message.

Deno

import { Cron } from "https://deno.land/x/croner@6.0.0/mod.ts";

const cron = new Cron("*/1 * * * *", () => {
  console.log("Task runs every minute.");
});

cron.start();

// Keep the program running
setTimeout(() => {
  cron.stop();
  console.log("Cron stopped after 3 minutes.");
}, 180000);

Important Notes

Make sure your program keeps running to allow scheduled tasks to execute.

You can stop a cron job anytime with cron.stop().

Cron expressions have 5 parts: minute, hour, day of month, month, day of week.

Summary

Scheduled tasks automate running code at set times.

Deno uses the croner library to create cron jobs.

Start tasks with cron.start() and stop with cron.stop().

Practice

1. What is the main purpose of using scheduled tasks (cron) in Deno?

easy

A. To compile Deno scripts faster

B. To manually trigger functions on user input

C. To debug code step-by-step

D. To run code automatically at specific times or intervals

Solution

1. Step 1: Understand scheduled tasks

   Scheduled tasks run code automatically without manual intervention.

2. Step 2: Identify cron job purpose in Deno

   Deno's cron jobs automate running code at set times or intervals.

3. Final Answer:

   To run code automatically at specific times or intervals -> Option D

4. Quick Check:

   Scheduled tasks = automatic timed code run [OK]

Hint: Scheduled tasks run code on a timer, not manually [OK]

Common Mistakes:

• Confusing scheduled tasks with manual triggers
• Thinking cron is for debugging
• Assuming cron speeds up compilation

2. Which of the following is the correct way to import the croner library in a Deno script?

easy

A. import Cron from "croner";

B. require('croner');

C. import { Cron } from "https://deno.land/x/croner@6.0.0/mod.ts";

D. import * as Cron from 'cron';

Solution

1. Step 1: Check Deno import style

   Deno uses ES module imports with URLs, not CommonJS require.

2. Step 2: Identify correct croner import

   The official croner import uses a URL with version and named import.

3. Final Answer:

   import { Cron } from "https://deno.land/x/croner@6.0.0/mod.ts"; -> Option C

4. Quick Check:

   Deno imports use URLs and named imports [OK]

Hint: Deno imports use full URLs, not short names or require [OK]

Common Mistakes:

• Using require() which is Node.js style
• Importing from 'cron' instead of 'croner'
• Using wildcard import without URL

3. What will the following Deno croner code output when run?

import { Cron } from "https://deno.land/x/croner@6.0.0/mod.ts";
const cron = new Cron("* * * * *", () => {
  console.log("Hello every minute");
});
cron.start();

medium

A. Prints "Hello every minute" every second

B. Prints "Hello every minute" every minute

C. Prints "Hello every minute" once immediately

D. Throws a syntax error

Solution

1. Step 1: Understand the cron pattern "* * * * *"

   This pattern means the task runs every minute.

2. Step 2: Analyze the callback behavior

   The callback logs "Hello every minute" each time the cron triggers.

3. Final Answer:

   Prints "Hello every minute" every minute -> Option B

4. Quick Check:

   Cron pattern "* * * * *" = every minute [OK]

Hint: Five stars in cron means every minute [OK]

Common Mistakes:

• Thinking it runs every second
• Expecting immediate output without waiting
• Confusing cron syntax with other formats

4. Identify the error in this Deno croner code snippet:

import { Cron } from "https://deno.land/x/croner@6.0.0/mod.ts";
const cron = new Cron("60 * * * *", () => {
  console.log("This runs every hour");
});
cron.start();

medium

A. The cron pattern "60 * * * *" is invalid because minutes must be 0-59

B. Missing await before cron.start()

C. Callback function should be async

D. Cron library does not support hourly tasks

Solution

1. Step 1: Check the cron pattern validity

   Minutes field must be between 0 and 59; "60" is invalid.

2. Step 2: Confirm other code parts

   cron.start() does not require await; callback can be sync; hourly tasks are supported.

3. Final Answer:

   The cron pattern "60 * * * *" is invalid because minutes must be 0-59 -> Option A

4. Quick Check:

   Minutes range = 0-59, 60 is invalid [OK]

Hint: Minutes in cron must be 0-59, not 60 or above [OK]

Common Mistakes:

• Using invalid minute value 60
• Thinking cron.start() needs await
• Assuming callback must be async

5. You want to schedule a Deno task to run every weekday at 9:30 AM using croner. Which cron pattern is correct?

hard

A. 30 9 * * 1-5

B. 9 30 * * 1-5

C. 30 9 * * 6,7

D. 0 9 * * 1-5

Solution

1. Step 1: Understand cron fields order

   Cron fields are: minute, hour, day of month, month, day of week.

2. Step 2: Match 9:30 AM on weekdays

   Minute = 30, hour = 9, day of week = 1-5 (Monday to Friday).

3. Final Answer:

   "30 9 * * 1-5" -> Option A

4. Quick Check:

   Minute=30, Hour=9, Weekdays=1-5 [OK]

Hint: Minute first, hour second, weekdays 1-5 for Mon-Fri [OK]

Common Mistakes:

• Swapping hour and minute fields
• Using weekend days 6,7 instead of weekdays
• Setting minute or hour to zero incorrectly