domain or business logic

Here are a few of the questions you should ask

when writing business logic:

¡Do you fully understand the business rule you’re implementing?

¡Are there any edge cases or exceptions that you need to cover?

¡How can you prove that your implementation is correct?

¡How easy will it be to change your code if the business rules change?

¡Will you, or someone else, understand the code if it needs changing later?