javascript - 使用 `` `for...in ``` 实现 combineReducers？

Question

在 egghead.io series on Redux 的第 16 课中，在看 Dan 是如何做到的之前，我尝试实现了自己的 combineReducers 函数。我得到了以下。我尝试在像这样传入的子 reducer (todos、visibilityFilter)上使用 for ... in

const combineReducers  = (reducers) => {
   return (state,action) => {
    let temp = {};
    for (let i in reducers) {
      temp[i] =  reducers[i](state,action)
    }
    return temp;

  }
}

这是行不通的。当我使用 expect 库对其进行测试时，我在控制台中收到以下错误。奇怪的是，如果我没记错的话，调用 todos reducer 的状态似乎已嵌套到 visibilityFilter reducer 的调用中。这很奇怪，因为我的代码显示它们在返回的对象中是截然不同的字段。

Uncaught Error: Expected { todos: [ { completed: false, id: 1, text: 'Go shopping' } ], visibilityFilter: { todos: [ { completed: false, id: 0, text: 'Learn Redux' } ], visibilityFilter: 'SHOW_ALL' } } to equal { todos: [ { completed: false, id: 0, text: 'Learn Redux' }, { completed: false, id: 1, text: 'Go shopping' } ], visibilityFilter: 'SHOW_ALL' }

我的测试代码是

const testTodoApp = () => {
  const stateBefore = {
    todos: [{id: 0, text:'Learn Redux', completed: false}],
    visibilityFilter: 'SHOW_ALL',
  };

  // action is an object. with a defined type property.
  const action = {
    type: 'ADD_TODO',
    id: 1,
    text: 'Go shopping',
  };

  const stateAfter = {
    todos: [{id: 0, text:'Learn Redux', completed: false},
            {id: 1, text:'Go shopping', completed: false},
            ],
    visibilityFilter: 'SHOW_ALL',
  };

  deepFreeze(stateBefore);
  deepFreeze(action);

  expect(
    todoApp(stateBefore, action)
  ).toEqual(stateAfter);
  console.log("Test passed: todoApp")

}

testTodoApp();

如果我使用内置的 combineReducers，这个测试将会通过。子 reducer 和对 combineReducers 的调用如下:

const todo = (state = {} ,action) => {
  switch (action.type) {

    case 'ADD_TODO':
    return {
      id: action.id, text: action.text, completed: false,
    };

    case 'TOGGLE_TODO':
    if (state.id !== action.id) {
      return state;
    }
    return {
      ...state, completed: !state.completed,
    };

    default:
    return state;

  }

}
const todos = (state=[], action) =>{

  switch (action.type) {
    case 'ADD_TODO':
    console.log('ADD_TODO switch selected')
    return [
      ...state,
      todo(undefined,action),
    ];

    case 'TOGGLE_TODO':
    console.log('TOGGLE_TODO switch selected')

    return state.map( t => todo(t, action))

    default:
    console.log('default switch selected')
    return state;
  }
}

const visibilityFilter = (
  state = 'SHOW_ALL',
  action
) => {
  switch (action.type) {
    case 'SET_VISIBILITY_FILTER':
      return action.filter;
    default:
      return state;
  }
}


const todoApp = combineReducers({
  todos,
  visibilityFilter,
})

我的问题是:

1. 在我的代码中是什么导致了一个 reducer 嵌套在另一个 reducer 中？
2. 我知道 Dan​​ 使用的是 reduce，但出于教学原因，我该如何使用 for ... in 模式来实现 combineReducers？
3. 之后，您能否评论一下对此类应用程序使用 for ... in 的适当性，如果它是一个糟糕的模式，是什么让它如此？

Answer 1

我刚刚意识到 todos reducer 和 visibilityFilter reducer 必须传递与它们的键对应的组合状态部分，不是整个组合状态。所以工作代码应该是这样的，我在第 5 行向状态的相应部分添加了一个对象访问器。

const combineReducers  = (reducers) => {
   return (state,action) => {
    let temp = {};
    for (let i in reducers) {
      temp[i] =  reducers[i](state[i],action)
    }
    return temp;

  }
}