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Scheduling Tasks in Node.js with node-cron

Cron is (one of || the) most famous job scheduler out there. It is used in Unix-based operating systems.
Let’s say you have a very susceptible computer. You want to be sure you don’t forget its birthday. If we want to run echo "I love you 💻" every year at a specific date (this will output ‘I love you 💻’ in whatever environment you scheduled it in).
Here is what it would look like in cron (try to guess when is my computer’s birthday).
# ┌───────────── minute (0 - 59)
# │  ┌───────────── hour (0 - 23)
# │  │ ┌───────────── day of the month (1 - 31)
# │  │ │  ┌───────────── month (1 - 12)
# │  │ │  │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday;
# │  │ │  │ │                                   7 is also Sunday on some systems)
# │  │ │  │ │
# │  │ │  │ │
# 10 8 23 9 * echo "I love you 💻"
Every year, the 23rd of September at 8:10am the computer will get a lovely message. Please note that the date is military so if we want to celebrate the birthday at 8pm we would write:
# 10 20 23 9 * echo "I love you 💻"
The wildcard sign * means any. In this example it means any day of the week. If we wrote:
# 10 20 23 9 1 echo "I love you 💻"
It would only wish a happy birthday when the computer’s birthday is on a Monday. The wild card can be used anywhere. To run a command every hour we would do: 0 * * * *. You can translate that command into “Run command when minute === 0”. Every hour on Monday would be 0 * * * 1.
For our apps the cron jobs will be:
# Morning 💊
30 8 * * *
# Afternoon 💊
30 15 * * *
# Night 💊
15 23 * * *

Using node-cron

I didn’t choose node-cron over other Node.js cron-like libraries for any other reason that it works for what I need.
To register a task all we have to do is:
const cron = require('node-cron');
const sendBirthdayMessage = () => { console.log("I love you 💻") };

// schedule takes two arguments, cron time and the task to call when we reach that time
cron.schedule('10 20 23 9 1', sendBirthdayMessage())
Let’s incorporate it in our app.
// Each subscription is connected to a client
const allSubscriptions = {}

const registerTasks = (subscription) => {
  const endpoint = subscription.endpoint;

  // My remote machine is on UTC time and I live in Japan so I made the appropriate calculation
  const morningTask = cron.schedule('30 23 * * *', () => {
    sendNotification(subscription, JSON.stringify({ timeOfDay: 'morning' }));
  });
  const afternoonTask = cron.schedule('30 6 * * *', () => {
    sendNotification(subscription, JSON.stringify({ timeOfDay: 'afternoon' }));
  });
  const nightTask = cron.schedule('15 14 * * *', () => {
    sendNotification(subscription, JSON.stringify({ timeOfDay: 'evening' }));
  });

  allSubscriptions[endpoint] = [morningTask, afternoonTask, nightTask];
};

app.post('/subscribe', (req, res) => {
  const subscription = req.body;
  registerTasks(subscription);
  res.send('subscribed!');
});


// Allows our client to unsubscribe
app.post('/unsubscribe', (req, res) => {
  const endpoint = req.body.endpoint;
  allSubscriptions[endpoint].forEach(task => {
    // destroy allows us to delete a task
    task.destroy();
  });
  delete allSubscriptions[endpoint];
});

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