递归函数在express js中没有返回值？

Question

我在电子商务网站工作。我已经为所有类别创建了一个猫鼬模型。但包括作为子类别的类别的父 ID。 当收到请求发送时，所有类别都是从数据库中找到的。很好，但我想以嵌套的方式将类别发送给客户端。这就是我这样写的原因。

// This is mongoose model 


const mongoose = require('mongoose');

const categorySchema = new mongoose.Schema({
    name: {
        type: String,
        required: true,
        trim: true,
        unique: true
    },
    slug: {
        type: String,
        required: true,
        unique: true
    },
    parentId: {
        type: mongoose.Schema.Types.ObjectId
    }

}, { timestamps: true })

module.exports = mongoose.model("Category", categorySchema)



//   This is router


Router.get('/categories', async (req, res) => {


    const createCategory = (categories, parentId = null) => {

        const categoryList = []
        let category

        if (parentId === null) {
            category = categories.filter(cat => cat.parentId === undefined);
        } else {
            category = categories.filter(cat => cat.parentId === parentId)
        }

        category.map((cate) => {
            categoryList.push({
                _id: cate._id,
                name: cate.name,
                slug: cate.slug,
                children: createCategory(categories, cate._id)
            })
        })


        return categoryList
    }

    try {

        const categories = await Category.find({})

        const categoryList = createCategory(categories)

        res.status(200).json({ categoryList })

    } catch (error) {

        res.status(500).json(error)
    }
})

  // the data of databess
  
  [
  {
    _id: new ObjectId("613c4ae2ff0098e41b1ae89a"),
    name: 'Mobile',
    slug: 'Mobile',
    createdAt: 2021-09-11T06:21:22.137Z,
    updatedAt: 2021-09-11T06:21:22.137Z,
    __v: 0
  },
  {
    _id: new ObjectId("613c4b11ff0098e41b1ae89c"),
    name: 'Oppo',
    slug: 'Oppo',
    parentId: '613c4ae2ff0098e41b1ae89a',
    createdAt: 2021-09-11T06:22:09.068Z,
    updatedAt: 2021-09-11T06:22:09.068Z,
    __v: 0
  },
  {
    _id: new ObjectId("613c4b21ff0098e41b1ae89e"),
    name: 'Samsung',
    slug: 'Samsung',
    parentId: '613c4ae2ff0098e41b1ae89a',
    createdAt: 2021-09-11T06:22:25.359Z,
    updatedAt: 2021-09-11T06:22:25.359Z,
    __v: 0
  },
  {
    _id: new ObjectId("613c4b2fff0098e41b1ae8a0"),
    name: 'Nokia',
    slug: 'Nokia',
    parentId: '613c4ae2ff0098e41b1ae89a',
    createdAt: 2021-09-11T06:22:39.048Z,
    updatedAt: 2021-09-11T06:22:39.048Z,
    __v: 0
  },
  {
    _id: new ObjectId("613c4b47ff0098e41b1ae8a2"),
    name: 'Nokia 2.4',
    slug: 'Nokia-2.4',
    parentId: '613c4ae2ff0098e41b1ae89a',
    createdAt: 2021-09-11T06:23:03.580Z,
    updatedAt: 2021-09-11T06:23:03.580Z,
    __v: 0
  },
  {
    _id: new ObjectId("613c4b6cff0098e41b1ae8a4"),
    name: 'TV',
    slug: 'TV',
    createdAt: 2021-09-11T06:23:40.550Z,
    updatedAt: 2021-09-11T06:23:40.550Z,
    __v: 0
  },
  {
    _id: new ObjectId("613c4b7eff0098e41b1ae8a6"),
    name: 'Refrijerator',
    slug: 'Refrijerator',
    createdAt: 2021-09-11T06:23:58.782Z,
    updatedAt: 2021-09-11T06:23:58.782Z,
    __v: 0
  }
]

他们的问题是当我试图从客户端获取数据时，只有根级别的类别正在输出，它没有父 ID，但子数组是这样的空。

{
    "categoryList": [
        {
            "_id": "613c4ae2ff0098e41b1ae89a",
            "name": "Mobile",
            "slug": "Mobile",
            "children": []
        },
        {
            "_id": "613c4b6cff0098e41b1ae8a4",
            "name": "TV",
            "slug": "TV",
            "children": []
        },
        {
            "_id": "613c4b7eff0098e41b1ae8a6",
            "name": "Refrijerator",
            "slug": "Refrijerator",
            "children": []
        }
    ]
}

我认为“createCategory”递归函数不起作用。

请任何人帮助我，我是这个领域的新手......

Answer 1

由于_id 属性是ObjectId 的实例，它们永远不会与parentId 匹配，后者是字符串。

根据documentation：

ObjectId()具有以下属性和方法：

Attribute/Method	Description
str	Returns the hexadecimal string representation of the object.
...	...

所以改变这一行：

        children: createCategory(categories, cate._id)

到：

        children: createCategory(categories, cate._id.str)

备注

您对.map 的使用是一种反模式。您应该获取它的返回值，而不是在回调中使用push。替换这两个语句：

    const categoryList = []
    /* .. */
    category.map((cate) => {
        categoryList.push({
            _id: cate._id,
            name: cate.name,
            slug: cate.slug,
            children: createCategory(categories, cate._id.str)
        })
    })

...用这个单一的语句：

    const categoryList = category.map((cate) => ({
        _id: cate._id,
        name: cate.name,
        slug: cate.slug,
        children: createCategory(categories, cate._id.str)
    }))

其次，您可以通过不为parentId 参数提供默认值来避免函数开头的代码重复。所以这个函数可以变得更加紧凑：

const createCategory = (categories, parentId) => {
    const category = categories.filter(cat => cat.parentId === parentId);
    return category.map((cate) => ({
        _id: cate._id,
        name: cate.name,
        slug: cate.slug,
        children: createCategory(categories, cate._id.str)
    }));
};

Answer 2

修改下面的代码

 children: createCategory(categories, cate._id)

收件人：

children: createCategory(categories, cate._id.toString())