Learning D3.js 5 Mapping - Second Edition

Preventing side effects

This basically means that each small function should not save some state outside itself. Try to limit the use of global variables as much as possible and think of each function as  the following process:

1. Data arrives.
2. Perform some operations on the data.
3. Return results.

This way we can easily test each function independently, without worrying about the effect it has on the global state of the program.