Modular Design in JavaScript (Functional Programming Edition)

Jason Gorman demonstrates how to apply the principles of Modular Design in functional JavaScript.

Modules should:

1. Do one job
2. Hide their inner workings (e.g., data)
3. Have swappable dependencies
4. Only expose what clients need to use

[jason_v]

It's critical to point out that separation of the area calculation is only justified because your knowledge of the business domain makes it certain that a rigid length x width calculation is insufficient. In the absence of that business domain expertise or of that certainty, I would consider it perfectly reasonable to leave it in place (YAGNI).