无法理解以下代码中的filter和findIndex？在react-redux reducer函数中

const initialState = {
  todos: [
    { id: 1, text: 'Learn React', completed: false },
    { id: 2, text: 'Redux', completed: true },
    { id: 3, text: 'Write code', completed: false }
  ]
};

const todosReducer = (state = initialState, action) => {
  switch (action.type) {
    case 'DELETE_TODO':
      return {
        ...state,
        todos: state.todos.filter(todo => todo.id !== action.payload)
      };
    default:
      return state;
  }
};

const initialState = {
  todos: [
    { id: 1, text: 'Learn React', completed: false },
    { id: 2, text: 'Redux', completed: true },
    { id: 3, text: 'Write code', completed: false }
  ]
};

const todosReducer = (state = initialState, action) => {
  switch (action.type) {
    case 'TOGGLE_TODO':
      const index = state.todos.findIndex(todo => todo.id === action.payload);
      if (index !== -1) {
        const updatedTodos = [...state.todos];
        updatedTodos[index].completed = !updatedTodos[index].completed;
        return {
          ...state,
          todos: updatedTodos
        };
      }
      return state;
    default:
      return state;
  }
};