useApp.js

const reducer = (state, action) => {
  switch (action.type) {
    case 'add-friend':
      return {
        ...state,
        friends: [...state.friends, action.friend],
        history: [...state.history, state],
      }
    case 'undo': {
      const isEmpty = !state.history.length
      if (isEmpty) return state
      return { ...state.history[state.history.length - 1] }
    }
    default:
      return state
  }
}