Scheduled tasks (cron) in Deno - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a cron job in the context of scheduled tasks?

A cron job is a scheduled task that runs automatically at specified times or intervals, like a reminder that triggers actions without manual start.

beginner

How do you import the cron module in Deno for scheduling tasks?

You import it using: <code>import { Cron } from "https://deno.land/x/cron@v1.2.0/mod.ts";</code> to access cron scheduling features.

intermediate

What does the cron expression "*/5 * * * *" mean?

It means the task runs every 5 minutes, at minute 0, 5, 10, 15, and so on, every hour of every day.

beginner

How do you start a cron job in Deno after creating it?

You call the start() method on the Cron instance, like cron.start();, to begin running the scheduled tasks.

intermediate

Why is it important to stop a cron job when it is no longer needed?

Stopping a cron job frees resources and prevents unwanted tasks from running, helping keep your application efficient and predictable.

Which Deno module is commonly used for cron scheduling?

A https://deno.land/x/cron

B https://deno.land/std/http

C https://deno.land/x/fs

D https://deno.land/x/db

The cron module at https://deno.land/x/cron provides scheduling features for tasks.

What does the cron expression "0 0 * * *" schedule?

A Every hour

B Every day at midnight

C Every minute

D Every Sunday

The expression means the task runs daily at 00:00 (midnight).

How do you define the function to run on a cron schedule in Deno?

A Call it manually inside the cron module

B Use setTimeout inside the cron module

C Pass it as a callback to the Cron constructor

D Define it in a separate file only

You provide the function as a callback when creating the Cron instance.

What method stops a running cron job in Deno?

A stop()

B pause()

C end()

D cancel()

The stop() method halts the cron job from running further.

Which of these is NOT a valid part of a cron expression?

A Minute

B Month

C Day of week

D Year

Standard cron expressions do not include a year field.

Explain how to create and start a scheduled task using cron in Deno.

Think about the steps from import to running the task.

Describe the meaning of a cron expression and how it controls task timing.

Imagine setting an alarm clock for specific times.

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