Scheduled tasks (cron) in Deno - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the cron scheduler from the Deno standard library.

Deno

import { [1] } from "https://deno.land/std@0.203.0/cron/mod.ts";

Drag options to blanks, or click blank then click option'

Aschedule

BCron

Ccron

DCronJob

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase 'cron' instead of 'Cron'.

Importing a non-existent 'schedule' or 'CronJob' from the module.

The Cron class is imported from the Deno cron module to create scheduled tasks.

2fill in blank

medium

Complete the code to create a cron job that runs every minute.

Deno

const cron = new Cron({
  cronTime: "[1]",
  onTick: () => console.log("Task running every minute"),
});

Drag options to blanks, or click blank then click option'

A"0 * * * * *"

B"* * * * *"

C"*/1 * * * *"

D"* * * * * *"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 5-field cron expressions instead of 6-field for Deno.

Confusing the position of seconds and minutes.

Deno's cron module uses 6-field expressions (second, minute, hour, day, month, weekday). The expression 0 * * * * * runs the task once every minute at second 0.

3fill in blank

hard

Fix the error in the code to start the cron job.

Deno

cron.[1]();

Drag options to blanks, or click blank then click option'

Astart

Brun

Cexecute

Dbegin

Attempts:

3 left

💡 Hint

Common Mistakes

Using run() or execute() which do not exist.

Trying to call begin() which is not a method.

The start() method is used to begin the cron job in Deno's cron module.

4fill in blank

hard

Fill both blanks to create a cron job that runs every day at 7:30 AM and logs a message.

Deno

const dailyJob = new Cron({
  cronTime: "[1]",
  onTick: () => console.[2]("Good morning! Time to start your day."),
});

Drag options to blanks, or click blank then click option'

A"0 30 7 * * *"

Blog

Cinfo

D"7 30 * * * *"

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping minutes and hours in the cron expression.

Using console.info instead of console.log.

The cron expression "0 30 7 * * *" runs the job at 7:30:00 AM every day (sec=0, min=30, hour=7). The console.log method outputs the message to the console.

5fill in blank

hard

Fill all three blanks to create a cron job that runs every Monday at 9:00 AM, logs a start message, and then stops itself.

Deno

const weeklyJob = new Cron({
  cronTime: "[1]",
  onTick: () => {
    console.[2]("Weekly job started.");
    weeklyJob.[3]();
  },
});

Drag options to blanks, or click blank then click option'

A"0 0 9 * * 1"

Blog

Cstop

D"0 9 * * 1"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 5-field cron expressions instead of 6-field.

Calling start() instead of stop() to end the job.

The cron expression "0 0 9 * * 1" runs the job at 9:00:00 AM every Monday. console.log prints the message, and stop() stops the cron job after running.

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