9 个JSON.stringify 的秘密大多数开发人员却不知道

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英文 | https://javascript.plainenglish.io/as-a-front-end-engineer-9-secrets-about-json-stringify-you-should-know-about-e71c175f40d8

作为前端开发工程师，你一定用过JSON.stringify，但你知道它的全部秘密吗？

很久以前，我因此在工作中犯下了无法挽回的错误。如果我早点知道，就不会发生这样的悲剧。

理解 JSON.stringify

基本上，JSON.stringify 将对象转换为 JSON 字符串。同时，JSON.stringify 有如下规则。

1. undefined、Function 和 Symbol 不是有效的 JSON 值。

如果在转换过程中遇到任何此类值，它们要么被省略（当在对象中找到时），要么更改为 null（当在数组中找到时）。

当传入像 JSON.stringify(function() {}) 或 JSON.stringify(undefined) 这样的“纯”值时，JSON.stringify() 可以返回 undefined。

2.Boolean、Number、String对象在字符串化过程中被转换为对应的原始值，符合传统的转换语义。

3.所有以符号为键的属性将被完全忽略，即使在使用替换函数时也是如此。

4. 数字 Infinity 和 NaN，以及值 null，都被认为是 null。

5. 如果该值有一个 toJSON() 方法，它负责定义哪些数据将被序列化。

6. Date实例通过返回字符串实现toJSON()函数（同date.toISOString()）。 

因此，它们被视为字符串。

7. 在包含循环引用的对象上执行此方法会抛出错误。

8. 所有其他对象实例（包括 Map、Set、WeakMap 和 WeakSet）将只序列化它们的可枚举属性。

9.尝试转换BigInt类型的值时抛出错误。

自己实现 JSON.stringify

理解功能的最好方法是自己去实现它。 下面我写了一个简单的函数来模拟JSON.stringify。

const jsonstringify = (data) => {
  // Check if an object has a circular reference
  const isCyclic = (obj) => {
  // Use the Set data type to store detected objects
  let stackSet = new Set()
  let detected = false

  const detect = (obj) => {
    // If it is not an object type, you can skip it directly
    if (obj && typeof obj != 'object') {
      return
    }
    // When the object to be checked already exists in the stackSet, it means that there is a circular reference
    if (stackSet.has(obj)) {
      return detected = true
    }
    // Save the current obj as a stackSet
    stackSet.add(obj)

    for (let key in obj) {
      if (obj.hasOwnProperty(key)) {
        detect(obj[key])
      }
    }
    // After the level detection is completed, delete the current object to prevent misjudgment
    /*
      For example:
      an object's attribute points to the same reference. 
      If it is not deleted, it will be regarded as a circular reference
      let tempObj = {
        name: 'fatfish'
      }
      let obj4 = {
        obj1: tempObj,
        obj2: tempObj
      }
    */
    stackSet.delete(obj)
  }

  detect(obj)

  return detected
}

  // 7#:
  // Executing this method on an object that contains a circular reference throws an error.

  if (isCyclic(data)) {
    throw new TypeError('Converting circular structure to JSON')
  }

  // 9#: An error is thrown when trying to convert a value of type BigInt
  // An error is thrown when trying to convert a value of type bigint
  if (typeof data === 'bigint') {
    throw new TypeError('Do not know how to serialize a BigInt')
  }

  const type = typeof data
  const commonKeys1 = ['undefined', 'function', 'symbol']
  const getType = (s) => {
    return Object.prototype.toString.call(s).replace(/\[object (.*?)\]/, '$1').toLowerCase()
  }

  // not an object
  if (type !== 'object' || data === null) {
    let result = data
    // 4#：The numbers Infinity and NaN, as well as the value null, are all considered null.
    if ([NaN, Infinity, null].includes(data)) {
      result = 'null'
      // 1#：undefined, Function, and Symbol are not valid JSON values. 
      // If any such values are encountered during conversion they are either omitted (when found in an object) or changed to null (when found in an array). 
      // JSON.stringify() can return undefined when passing in "pure" values like JSON.stringify(function() {}) or JSON.stringify(undefined).
    } else if (commonKeys1.includes(type)) {
      return undefined
    } else if (type === 'string') {
      result = '"' + data + '"'
    }

    return String(result)
  } else if (type === 'object') {
    // 5#: If the value has a toJSON() method, it's responsible to define what data will be serialized.
    // 6#: The instances of Date implement the toJSON() function by returning a string (the same as date.toISOString()). 
    // Thus, they are treated as strings.
    if (typeof data.toJSON === 'function') {
      return jsonstringify(data.toJSON())
    } else if (Array.isArray(data)) {
      let result = data.map((it) => {
        // 1#: If any such values are encountered during conversion they are either omitted (when found in an object) or changed to null (when found in an array). 
        return commonKeys1.includes(typeof it) ? 'null' : jsonstringify(it)
      })

      return `[${result}]`.replace(/'/g, '"')
    } else {
      // 2#：Boolean, Number, and String objects are converted to the corresponding primitive values during stringification, in accord with the traditional conversion semantics.
      if (['boolean', 'number'].includes(getType(data))) {
        return String(data)
      } else if (getType(data) === 'string') {
        return '"' + data + '"'
      } else {
        let result = []
        // 8#: All the other Object instances (including Map, Set, WeakMap, and WeakSet) will have only their enumerable properties serialized.
        Object.keys(data).forEach((key) => {
          // 3#: All Symbol-keyed properties will be completely ignored, even when using the replacer function.
          if (typeof key !== 'symbol') {
            const value = data[key]
            // 1#: undefined, Function, and Symbol are not valid JSON values.
            if (!commonKeys1.includes(typeof value)) {
              result.push(`"${key}":${jsonstringify(value)}`)
            }
          }
        })

        return `{${result}}`.replace(/'/, '"')
      }
    }
  }
}

还有一个测试

// 1. Test basic features
console.log(jsonstringify(undefined)) // undefined 
console.log(jsonstringify(() => { })) // undefined
console.log(jsonstringify(Symbol('fatfish'))) // undefined
console.log(jsonstringify((NaN))) // null
console.log(jsonstringify((Infinity))) // null
console.log(jsonstringify((null))) // null
console.log(jsonstringify({
  name: 'fatfish',
  toJSON() {
    return {
      name: 'fatfish2',
      sex: 'boy'
    }
  }
}))
// {"name":"fatfish2","sex":"boy"}

// 2. Compare with native JSON.stringify
console.log(jsonstringify(null) === JSON.stringify(null));
// true
console.log(jsonstringify(undefined) === JSON.stringify(undefined));
// true
console.log(jsonstringify(false) === JSON.stringify(false));
// true
console.log(jsonstringify(NaN) === JSON.stringify(NaN));
// true
console.log(jsonstringify(Infinity) === JSON.stringify(Infinity));
// true
let str = "fatfish";
console.log(jsonstringify(str) === JSON.stringify(str));
// true
let reg = new RegExp("\w");
console.log(jsonstringify(reg) === JSON.stringify(reg));
// true
let date = new Date();
console.log(jsonstringify(date) === JSON.stringify(date));
// true
let sym = Symbol('fatfish');
console.log(jsonstringify(sym) === JSON.stringify(sym));
// true
let array = [1, 2, 3];
console.log(jsonstringify(array) === JSON.stringify(array));
// true
let obj = {
  name: 'fatfish',
  age: 18,
  attr: ['coding', 123],
  date: new Date(),
  uni: Symbol(2),
  sayHi: function () {
    console.log("hello world")
  },
  info: {
    age: 16,
    intro: {
      money: undefined,
      job: null
    }
  },
  pakingObj: {
    boolean: new Boolean(false),
    string: new String('fatfish'),
    number: new Number(1),
  }
}
console.log(jsonstringify(obj) === JSON.stringify(obj)) 
// true
console.log((jsonstringify(obj)))
// {"name":"fatfish","age":18,"attr":["coding",123],"date":"2021-10-06T14:59:58.306Z","info":{"age":16,"intro":{"job":null}},"pakingObj":{"boolean":false,"string":"fatfish","number":1}}
console.log(JSON.stringify(obj))
// {"name":"fatfish","age":18,"attr":["coding",123],"date":"2021-10-06T14:59:58.306Z","info":{"age":16,"intro":{"job":null}},"pakingObj":{"boolean":false,"string":"fatfish","number":1}}

// 3. Test traversable objects
let enumerableObj = {}

Object.defineProperties(enumerableObj, {
  name: {
    value: 'fatfish',
    enumerable: true
  },
  sex: {
    value: 'boy',
    enumerable: false
  },
})

console.log(jsonstringify(enumerableObj))
// {"name":"fatfish"}

// 4. Testing circular references and Bigint

let obj1 = { a: 'aa' }
let obj2 = { name: 'fatfish', a: obj1, b: obj1 }
obj2.obj = obj2

console.log(jsonstringify(obj2))
// TypeError: Converting circular structure to JSON
console.log(jsonStringify(BigInt(1)))
// TypeError: Do not know how to serialize a BigInt

最后

以上就是我今天跟你分享的全部内容，希望你能从今天的文章中学到新的知识。

最后，感谢你的阅读，祝编程愉快！

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