Refactor existing code

[KlavierCat]

TODO:

• Naming: currently there are both Id and ID. We need to unify this, both in app.js and the faked DB. Function names can be improved to make more sense
• add comment, use Javadoc format.
• Stay DRY: try to abstract away anything that is repetitive in our current code
• if they are constants, do not use var, use const
• refactor long function. Functions shouldn't be too long and each function should do only one thing.
• Reorder: both consts / vars and function. In an order that makes more senses.
• If necessary, separate the code into different modules and use "require" to require them. Probably not necessary at this stage.