Observables and actions keep things within the confines of the MobX reactivity system. Actions mutate the observables and, through the power of notifications, the rest of the MobX system aligns to the mutation to keep the state consistent. To start making a change outside of this MobX system, you need reactions. It is the bridge to the outside world that informs the state-changes happening within the MobX world. 

Consider reactions to be the reactive-bridge-crossing between MobX and  the outside world. These are also the side effect producers of your application.

We know that reactions come in three flavors: autorun, reaction, and when. These three flavors have distinct characteristics that tackle the various scenarios within your app.

When determining which one to pick, you can apply this simple decision-making process:

Each of the reactions give you back a disposer-function, which can be used to prematurely dispose the reaction thus:

import { autorun, reaction, when } from 'mobx'...