persist_state.js

// Create an effect to persist data. For native envs, using something similar
// to localStorage
const persistedState = {
  set(state) {
    localStorage.setItem('state', JSON.stringify(state))
  },
  get() {
    return JSON.parse(localStorage.getItem('state') || '{}')
  }
}

// Use the "onInitialize" action that fires when Overmind initializes
import merge from 'deepmerge'

const onInitialize = ({ state, effects }, overmind) => {
  const initialState = effects.persistedState.get()
  
  // We use "deepmerge" as it will ensure that any use of "derived"
  // is still kept
  merge(state, initialState)
  
  // Every time Overmind flushes out changes to state we persist it
  overmind.addFlushListener(() => {
    effects.persistedState.set(state)
  })
  
  // OR: you might consider using a debounce around it, for performance
  // reasons
  overmind.addFlushListener(debounce(() => {
    effects.persistedState.set(state)
  }, 500))
}