next.js/examples/with-rematch/shared/models/counter.js

const counter = {
  state: 0, // initial state
  reducers: {
    // handle state changes with pure functions
    increment (state, payload) {
      return state + payload
    }
  },
  effects: {
    // handle state changes with impure functions.
    // use async/await for async actions
    async incrementAsync (payload, rootState) {
      await new Promise(resolve => setTimeout(resolve, 1000))
      this.increment(payload)
    }
  }
}

export default counter