Yann - Reducer

reducer.js

case 'TOGGLE_TODO':
  return state.map((todo) => {
    if (todo.id === action.id) {
      var nextCompleted = !todo.completed;

      return {
        ...todo,
        completed: nextCompleted,
        completedAt: nextCompleted ? moment().unix() : undefined
      };
    }
    
    return todo;
  });