之前学习爬虫的时候，如果是 HTML 的数据，通过 xpath 或是 css 选择器，就能很快的获取我们想要的数据，如果是 json 有没有类似 xpath 这种，能够直接根据条件定位数据，而不需要自行 json 解析在遍历获取。答案是有的，也就是 JSONPath。

在线测试网址 JSONPath 在线验证

所选用的环境是 Node + JavaScript，用到 jsonpath 这个包 jsonpath - npm (npmjs.com)

参考链接 JsonPath - 根据表达式路径解析 Json - 简书 (jianshu.com)

基本语法

过滤器表达式

通常的表达式格式为：[?(@.age > 18)] 表示当前节点属性 age 大于 18

操作符	描述
==	等于符号，但数字 1 不等于字符 1(note that 1 is not equal to ‘1’)
!=	不等于符号
<	小于符号
<=	小于等于符号
>	大于符号
>=	大于等于符号
=~	判断是否符合正则表达式，例如[?(@.name =~ /foo.*?/i)]
in	所属符号，例如[?(@.size in [‘S’, ‘M’])]
nin	排除符号
size	size of left (array or string) should match right
empty	判空 Null 符号

语法就这些，不过单单有语法，不实践肯定是不够的。下面就是一些官方简单例子操作，还有一个终极实战

代码演示

js
var jp = require('jsonpath')

var cities = [
  { name: 'London', population: 8615246 },
  { name: 'Berlin', population: 3517424 },
  { name: 'Madrid', population: 3165235 },
  { name: 'Rome', population: 2870528 },
]

var names = jp.query(cities, '$..name')

// [ "London", "Berlin", "Madrid", "Rome" ]

如果使用 js 来遍历的话，也简单

js
let names = cities.map(c => c.name)

这个数据可能还没那么复杂，在看看下面这个例子，代码来源于https://goessner.net/articles/JsonPath

json
{
  "store": {
    "book": [
      {
        "category": "reference",
        "author": "Nigel Rees",
        "title": "Sayings of the Century",
        "price": 8.95
      },
      {
        "category": "fiction",
        "author": "Evelyn Waugh",
        "title": "Sword of Honour",
        "price": 12.99
      },
      {
        "category": "fiction",
        "author": "Herman Melville",
        "title": "Moby Dick",
        "isbn": "0-553-21311-3",
        "price": 8.99
      },
      {
        "category": "fiction",
        "author": "J. R. R. Tolkien",
        "title": "The Lord of the Rings",
        "isbn": "0-395-19395-8",
        "price": 22.99
      }
    ],
    "bicycle": {
      "color": "red",
      "price": 19.95
    }
  }
}

JsonPath	Result
$.store.book[*].author	所有 book 的 author 节点
$..author	所有 author 节点
$.store.*	store 下的所有节点，book 数组和 bicycle 节点
$.store..price	store 下的所有 price 节点
$..book[2]	匹配第 3 个 book 节点
$..book[(@.length-1)]，或 $..book[-1:]	匹配倒数第 1 个 book 节点
$..book[0,1]，或 $..book[:2]	匹配前两个 book 节点
$..book[?(@.isbn)]	过滤含 isbn 字段的节点
$..book[?(@.price<10)]	过滤price<10的节点
$..*	递归匹配所有子节点

对应的语法可直接到在 JSONPath 在线验证网站上进行测试。要提一点的是，jsonpath 是支持使用 || 与 && 进行过滤的，比如上面要获取 category 为 fiction，price 大于 10 的语法为$..book[?(@.price>10 && @.category=="fiction")] 结果如下

json
[
  {
    "category": "fiction",
    "author": "Evelyn Waugh",
    "title": "Sword of Honour",
    "price": 12.99
  },
  {
    "category": "fiction",
    "author": "J. R. R. Tolkien",
    "title": "The Lord of the Rings",
    "isbn": "0-395-19395-8",
    "price": 22.99
  }
]

终极实战

也许你会觉得上面的例子太过简单了，可能没达到你预期所想要的效果，甚至还不如使用 json 遍历呢，下面我列举一个是我实战中遇到的例子（实际上这样的例子特别多），我先把部分数据展示出来（删除部分没用到的参数，实际参数远比这多），然后通过 js 遍历，以及 jsonpath 来获取我想要的数据。

结构

数据

json
{
  "role": "unit",
  "children": [
    {
      "role": "section",
      "children": [
        {
          "role": "node",
          "children": [
            {
              "summary": "{\"indexMap\": {}, \"questionsList\": []}",
              "role": "group",
              "tab_type": "text",
              "name": "Learning objectives",
              "scoreDetail": [],
              "id": "u1g2",
              "url": "u1g2",
              "tags": []
            }
          ],
          "suggestedDuration": "0",
          "name": "1-1 Learning objectives",
          "block_id": "90ed499f91084e2aa1b7032d2e4ecd76",
          "url": "u1g1",
          "tags": []
        }
      ],
      "name": "Learning objectives",
      "tags": []
    },
    {
      "role": "section",
      "children": [
        {
          "role": "node",
          "children": [
            {
              "role": "node",
              "children": [
                {
                  "role": "group",
                  "tab_type": "task",
                  "name": "Practice-1",
                  "scoreDetail": [0],
                  "id": "u1g6",
                  "url": "u1g6",
                  "tags": []
                },
                {
                  "role": "group",
                  "tab_type": "task",
                  "name": "Practice-2",
                  "scoreDetail": [1, 1, 1, 1],
                  "id": "u1g7",
                  "url": "u1g7",
                  "tags": []
                },
                {
                  "role": "group",
                  "tab_type": "task",
                  "name": "Practice-3",
                  "scoreDetail": [1, 1, 1, 1, 1],
                  "id": "u1g544",
                  "url": "u1g544",
                  "tags": []
                },
                {
                  "role": "group",
                  "tab_type": "task",
                  "name": "Practice-4",
                  "scoreDetail": [1, 1, 1, 1, 1],
                  "id": "u1g9",
                  "url": "u1g9",
                  "tags": []
                }
              ],
              "name": "Practice",
              "block_id": "f6768dc9474746b9ba071e7f211534d9",
              "url": "u1g5",
              "tags": []
            }
          ],
          "suggestedDuration": "0",
          "name": "1-2 Sharing",
          "block_id": "1c97a87a9feb4a8aa7d6ed39482d866d",
          "url": "u1g3",
          "tags": []
        },
        {
          "role": "node",
          "children": [
            {
              "role": "group",
              "tab_type": "video",
              "name": "Get the skills",
              "scoreDetail": [],
              "id": "u1g16",
              "url": "u1g16",
              "tags": []
            },
            {
              "role": "node",
              "children": [
                {
                  "role": "group",
                  "tab_type": "task",
                  "name": "Use the skills-1",
                  "scoreDetail": [0, 0],
                  "id": "u1g615",
                  "url": "u1g615",
                  "tags": []
                },
                {
                  "role": "group",
                  "tab_type": "task",
                  "name": "Use the skills-2",
                  "scoreDetail": [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
                  "id": "u1g18",
                  "url": "u1g18",
                  "tags": []
                },
                {
                  "role": "group",
                  "tab_type": "task",
                  "name": "Use the skills-3",
                  "scoreDetail": [0, 0, 0, 0, 0, 0, 0],
                  "id": "u1g19",
                  "url": "u1g19",
                  "tags": []
                }
              ],
              "name": "Use the skills",
              "block_id": "2d8a81799bcc44ccab2646b613557b2b",
              "url": "u1g17",
              "tags": []
            },
            {
              "role": "node",
              "children": [
                {
                  "role": "group",
                  "tab_type": "task",
                  "name": "Think and speak",
                  "scoreDetail": [0],
                  "id": "u1g21",
                  "url": "u1g21",
                  "tags": []
                }
              ],
              "name": "Think and speak",
              "block_id": "5833925c8c5e4ddab7a114b15d610983",
              "url": "u1g20",
              "tags": []
            }
          ],
          "suggestedDuration": "0",
          "name": "1-3 Listening",
          "block_id": "681817aaf75845468e464e1a8d82f2c8",
          "url": "u1g14",
          "tags": []
        },
        {
          "role": "node",
          "children": [
            {
              "role": "node",
              "children": [
                {
                  "role": "group",
                  "tab_type": "task",
                  "name": "Get a clue",
                  "scoreDetail": [0, 0, 0],
                  "id": "u1g25",
                  "url": "u1g25",
                  "tags": []
                }
              ],
              "name": "Get a clue",
              "block_id": "b332335ab3554dffb92afcae5f815b5a",
              "url": "u1g24",
              "tags": []
            },
            {
              "role": "node",
              "children": [
                {
                  "role": "group",
                  "tab_type": "task",
                  "name": "View it-1",
                  "scoreDetail": [1, 1, 1, 1],
                  "id": "u1g27",
                  "url": "u1g27",
                  "tags": []
                },
                {
                  "role": "group",
                  "tab_type": "task",
                  "name": "View it-2",
                  "scoreDetail": [1, 1, 1, 1, 1, 1, 1],
                  "id": "u1g545",
                  "url": "u1g545",
                  "tags": []
                },
                {
                  "role": "group",
                  "tab_type": "task",
                  "name": "View it-3",
                  "scoreDetail": [1, 1, 1, 1, 1, 1, 1, 1],
                  "id": "u1g29",
                  "url": "u1g29",
                  "tags": []
                }
              ],
              "name": "View it",
              "block_id": "a2ecf6464d5f480e98242ebe4431a73b",
              "url": "u1g26",
              "tags": []
            },
            {
              "role": "node",
              "children": [
                {
                  "role": "group",
                  "tab_type": "task",
                  "name": "Think and speak",
                  "scoreDetail": [0, 0],
                  "id": "u1g31",
                  "url": "u1g31",
                  "tags": []
                }
              ],
              "name": "Think and speak",
              "block_id": "120d0784e63c414793f5e648c416144b",
              "url": "u1g30",
              "tags": []
            }
          ],
          "suggestedDuration": "0",
          "name": "1-4 Viewing",
          "block_id": "765a1be83ac5437aaca8fa150ad5af2e",
          "url": "u1g22",
          "tags": []
        }
      ],
      "name": "Listening to the world",
      "tags": []
    }
  ]
}

需求

可以看到数据比一开始的例子复杂了可不是一点，不过先别管这些数据是干啥的，说说需求，从结构上也能看出来，是有很多children嵌套的，而需求就是获取role为group的children节点数据

js 实现遍历

先说说 js 如何实现的，我贴一下对应的代码（当时项目的代码，稍微修改的一点），可自己粘贴运行一下。

js
let groupList = []
for (const node of json.children ?? []) {
  if (node.role == 'group') groupList.push({ ...node })

  for (const group of node.children ?? []) {
    if (group.role == 'group') groupList.push({ ...group })

    for (const child of group.children ?? []) {
      if (child.role == 'group') groupList.push({ ...child })

      let children4 = child.children ?? []
      for (const child of children4) {
        if (child.role == 'group') groupList.push({ ...child })
      }
    }
  }
}
console.log(groupList)

因为这些数据中，是存在不确定性的，也就是在当前节点下，二级节点可能有children，而其他节点下的二级很可能没有 children，所以我在这边就加上 ?? [] （Typescript 中的??语法，你可以把 ?? 当做 || ）来判断是否有children节点，有些读者可能会思考，为啥不用递归呢。说的是挺轻松的，但是递归是很容易出问题的，万一爬取到后台数据进行了一些修改，很有可能对于的递归算法将失效，甚至导致堆栈溢出，所以我这边值循环 4 级chilren节点（实际遇到的貌似也只有 4 级，谁又能保证爬取到数据就一定只有 4 级呢）。

jsonpath 获取

于是了解到 jsonpath 后，我第一个时间就开始分析这样的数据，果不其然，得到了我想要的结果 ⬇️

语法：$..children[?(@.role=="group")]

语法意思很明确，根节点下遍历所有children节点，同时role等于group，呈现的效果如上图。

而回到需求，就是获取role为group的children节点数据，而 jsonpath 就帮我轻松实现我想要的效果。

最终思考

实际上这样的需求我已经不止遇到一次，二次了，然而我寻求百度与群友的时候，给我的结果都不尽人意。但都没有提及到 jsonpath 来进行获取。也许是我的搜索方式有问题，但千篇一律都是 js 如何解析多层 json，以及遍历所有的子元素，虽然这些办法确实能解决我的问题，但每次遇到这种数据，都需要花上长时间去编写对应的逻辑。

在回想起当时爬取 HTML 页面数据的时候（数据与上面展示的差不多，都是树结构多层），而我只接触到了正则表达式，没了解过 CSS 选择器与 xpath。怎么办，为了实现目的，只好用现有的技术去实现，于是编写一个正则表达式就花费了近一个下午的时间，而使用 CSS 选择器 10 分钟不到就达到目的。没想到竟然有这么好用的方法，早知道多去了解点技术了。可能现在的心情和当时一样，只不过 HTML 换成了 JSON，编辑器还是那个编辑器，而我依旧还是我

也许这就是编程，也许这就是人生。