Birthday Greetings

Technical challange solution for the Birthday Greetings kata.

Problem

As you’re a very friendly person, you would like to send a birthday note to all the friends you have. But you have a lot of friends and a bit lazy, it may take some times to write all the notes by hand.

The good news is that computers can do it automatically for you.

Imagine you have a flat file with all your friends :

last_name, first_name, date_of_birth, email
Doe, John, 1982/10/08, john.doe@foobar.com
Ann, Mary, 1975/09/11, mary.ann@foobar.com

The greetings email contains the following text:

Subject: Happy birthday!
Happy birthday, dear John!

Solution

As the purpose of this challange is to evaluate one's coding abilities, I've decided to keep things simple and not use any framework or huge library. There's a set of problems to solve:

How do you structure the code so it's easily extensible?

Notifiers and Stores are dynamically loaded from a configuration file, keeping execution time and memory footprint at a minimum. Adding a new notifier or store is as easy as creating a new handler (e.g. sms.js) and adding it to the configuration file.
How do you read and parse the flat file?

The flat file is read using a read stream and parsed by an "industry standard" CSV parser. While using a stream slightly increases the project complexity, it allows to handle huge files without running out of memory.
How do you send the email at the right time?

A cron expression is generated using the date of birth of each friend, which is then used to schedule the email sending.
```
const dob = new Date(row.date_of_birth)
const cronExpression = `0 0 0 ${dob.getDate()} ${dob.getMonth() + 1} * *`
```

Limitations

Keeping things simple, I've decided to not handle certain edge cases:

The date of birth validation is handled by JavaScript's built-in Date constructor and by a simple regex. Therefore there are many edge cases not accounted for. (as far as I could see, February 29th is correctly handled)
The server is stateless and doesn't keep track of emails sent. If there are crashes, it could potentially miss some emails or send the same email twice. This is a problem that could be easily fixed by adding some sort of DB or a log file.
The server is not restarted when the configuration file is updated or when new friends are added to the flat file.
There is no concurrency control, too many friends with the same birthday could cause problems.
At the moment, there's no way to easily deploy the project to a remote server, it's just meant to be run locally.

Development

Select the correct node version and install the required dependencies:

nvm use && npm i

Run the application in development mode:

npm run dev

Testing

Due to time constraints, only unit tests are currently implemented. Lots of improvements could be done, however, I feel that is out-of-scope for this project. You can run the unit tests using Jest:

npm test

Linting, Formatting and pre-commit hooks

The code is linted and automatically formatted by ESLint and Prettier. Husky and lint-staged are used to automatically run the linter and formatter before each commit.