What a great, enlightning post series! I’ve been searching for this kind of content for a while, and I’m glad you didn’t take the “easy” path and just provide theory with no down-to-earth examples.
As a side note, Vuex also has the concept of “Selectors” built on its core. They call them “Getters” (source), and they are just derived state based on store state.
Looking forward to reading part 4!