x.js

//array reducer
function todos(state = initialState, action) {
  switch (action.type) {
    case ADD_TODO:
      return [
        ...state,
        todo(null, action)
      ]
    case TOGGLE_TODO:
      return state.map(t => todo (t, action));
    default:
      return state
  }
}

function todo (state, action) {
   switch (action.type) {
    case ADD_TODO:
       return {
        text: action.text,
        completed, false
       }
    case TOGGLE_TODO:
      if (action.id === state.id) {
        return {
          ...state,
          completed: !state.completed
        }
      }
       
      return state;
  } 
}