Redux reducers: not to do

Reducers must be:

1. pure: pure reducer does not mutate the state directly.
2. side-effect free: reducer does not change a non-local state, nor use an external code. ex) console.log()
3. synchronous: reducer does not "await" for asynchronous results. ex) HTTP request

If side-effects & async tasks are necessary,

1. Use them inside components: via useEffect.
2. Use action creators.