Derivations (computed properties)

Derivation is a term that is used quite frequently in the MobX parlance. It is given special emphasis in client-state modeling. As we saw in the previous chapter, the observable state can be determined by the combination of the core-mutable-state and a derived-read-only-state:

Observable State = (Core-mutable-State) + (Derived-readonly-State)

It is essential to keep the core state as lean as possible. This is the part that is expected to stay stable and grow slowly during the lifetime of the application. It is only the core state that is actually mutable and the actions always mutate only the core state. The derived state depends on the core state and is kept up-to-date by the MobX reactivity system. We know that computed properties act as the derived state in MobX. They can depend not only on the core state but also on other derived states, creating a dependency tree that is kept alive by MobX:

A key characteristic of the derived state is that it is read only...