todo list reducer

reducer.js

const reducer = (state, event) => {
  switch (event.type) {
    case 'TodoAdded':
      // append event to the list of existing events.
      return [
        ...state,
        { title: event.title, completed: false },
      ]
 
     case 'TodoRemoved':
       // remove the item at given index (non-mutating).
       return [
          ...state.slice(0, event.index),
          ...state.slice(event.index + 1),
       ]
   
     case 'TodoCompleted':
       // change item at given index (non-mutating).
       return [
          ...state.slice(0, event.index),
          { ...state[event.index], completed: true },
          ...state.slice(event.index + 1),
       ]
     default:
       return state
  }
}