为什么 XMLHttpRequest 对象的属性只能通过 console.log() 打印？

Question

var obj = new XMLHttpRequest();
console.log("Object.keys():",Object.keys(obj));
console.log("Object.getOwnPropertyNames():",Object.getOwnPropertyNames(obj))
console.log("Object.entries():",Object.entries(obj))
console.log("JSON.stringify():",JSON.stringify(obj))

console.log("console.log:"); console.log(obj)

输出：

Object.keys(): []
Object.getOwnPropertyNames(): []
Object.entries(): []
JSON.stringify(): {}
console.log:
XMLHttpRequest {onreadystatechange: null, readyState: 0, timeout: 0, withCredentials: false, upload: XMLHttpRequestUpload, …}

如何在 javascript 中创建这样一个对象，其属性仅使用 console.log(obj) 打印，但不会由上述函数的 any 返回？

我已经尝试使用 constructor function、object.create()（使用 enumerable:false）、Object.assign()、使用 getters、从 class，从扩展 class e.t.c

实例化

Answer 1

你可以这样做：

Object.defineProperty(obj, 'key', {
    enumerable: false,
    value: '123'
});

具体来说，enumerable 东西隐藏了属性。

有关详细信息，请参阅此页面：https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty

更新：

以下方法（上述方法的改进版本）似乎隐藏了所有提到的检查的属性，尽管它有点冗长：

var proto = {};
Object.defineProperty(proto, "key", {
    get: function () { return 123; }
});

var obj = Object.create(proto);

console.log(obj); // {}
console.log(Object.getOwnPropertyNames(obj)); // []
console.log(obj.key); // 123

Answer 2

Object.keys：只给出对象自身的可枚举属性.

Object.getOwnPropertyNames：给出直接在给定对象中找到的可枚举和不可枚举属性。

Object.entries：给出对象自己的可枚举字符串键控属性对。

JSON.stringify：只考虑对象的可枚举属性。

创建您自己的此类对象

您可以使用 Object.create 创建一个新对象，使用现有对象作为新创建对象的原型。

obj1 = {
  key1: 'some key'
};

var obj = Object.create(obj1);


console.log("Object.keys():",Object.keys(obj));
console.log("Object.getOwnPropertyNames():",Object.getOwnPropertyNames(obj))
console.log("Object.entries():",Object.entries(obj))
console.log("JSON.stringify():",JSON.stringify(obj))

console.log("console.log:"); console.log(obj)

Answer 3

这是因为原型制作在Javascript中起作用，这才是真正的原因。

当您从原型创建对象实例时，__proto__ 中的属性将不会显示（不可枚举的属性）

例如，尝试：

function MyXMLHttpRequest(){};
MyXMLHttpRequest.prototype.name = "Edward"
var obj = new MyXMLHttpRequest();

console.log("Object.keys():",Object.keys(obj));
console.log("Object.getOwnPropertyNames():",Object.getOwnPropertyNames(obj))
console.log("Object.entries():",Object.entries(obj))
console.log("JSON.stringify():",JSON.stringify(obj))
console.log("console.log:"); console.log(obj)
console.log(obj.name)

您将看到：

Answer 4

您可以使用名为 Symbol 的新数据类型为 JavaScript 对象创建键。

Link: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol

我附上了示例代码，您可以在控制台中看到输出。

let nameSymbol = Symbol('name');
function Person (name) {
 this[nameSymbol] = name;
}

Person.prototype.toString = function toPersonString() {
 var symArr = Object.getOwnPropertySymbols(this);
 for (let sym of symArr) {
  this[sym.description] = this[sym];
  delete this[sym];
 }
 return this;
}

const dummyObject = new Person('random value');

console.log("Object.keys():",Object.keys(dummyObject)); // []
console.log("Object.getOwnPropertyNames():",Object.getOwnPropertyNames(dummyObject)) // []
console.log("Object.entries():",Object.entries(dummyObject)) // []
console.log("JSON.stringify():",JSON.stringify(dummyObject)) // {}

console.log("console.log:", dummyObject.toString()); //  {name: "random value"}

Answer 5

这与 WhatWG 有关 console.log specification:

How the implementation prints args is up to the implementation, but implementations should separate the objects by a space or something similar, as that has become a developer expectation.

规范使输出格式非常模糊，由实现决定打印什么。

Answer 6

Why properties of an XMLHttpRequest object are only printable through console.log()?

这不是真的

var obj = new XMLHttpRequest();
for(var key in obj) console.log(key);

根据描述回答问题

How can I create such an object in javascript, whose properties are only printed using

  var obj = Object.create({ foo: "foo", constructor: function() {} });

如果您想从 for in 中隐藏属性，您应该将 enumerable 属性设置为 false 以获得所需的属性:

Object.defineProperty(obj, 'foo', {
  value: 42,
  enumerable: false
});

Answer 7

我认为这是最实用的方法，对你有帮助

class User {
  //this is a private field
  #name
  get Name() {
    return this.#name
  }
  set Name(val) {
    this.#name = val
  }
}

var obj = new User()
obj.Name = 'test'

console.log('Object.keys():', Object.keys(obj))
console.log('Object.getOwnPropertyNames():', Object.getOwnPropertyNames(obj))
console.log('Object.entries():', Object.entries(obj))
console.log('JSON.stringify():', JSON.stringify(obj))

console.log('console.log:')
console.log(obj)
console.log(obj.Name)

这是编译后的代码

function _classPrivateFieldSet(receiver, privateMap, value) {
  var descriptor = privateMap.get(receiver)
  if (!descriptor) {
    throw new TypeError('attempted to set private field on non-instance')
  }
  if (descriptor.set) {
    descriptor.set.call(receiver, value)
  } else {
    if (!descriptor.writable) {
      throw new TypeError('attempted to set read only private field')
    }
    descriptor.value = value
  }
  return value
}
function _classPrivateFieldGet(receiver, privateMap) {
  var descriptor = privateMap.get(receiver)
  if (!descriptor) {
    throw new TypeError('attempted to get private field on non-instance')
  }
  if (descriptor.get) {
    return descriptor.get.call(receiver)
  }
  return descriptor.value
} // Try edit message
class User {
  constructor() {
    _name.set(this, { writable: true, value: void 0 })
  }
  get Name() {
    return _classPrivateFieldGet(this, _name)
  }
  set Name(val) {
    _classPrivateFieldSet(this, _name, val)
  }
}
var _name = new WeakMap()
var obj = new User()
obj.Name = 'test'
console.log('Object.keys():', Object.keys(obj))
console.log('Object.getOwnPropertyNames():', Object.getOwnPropertyNames(obj))
console.log('Object.entries():', Object.entries(obj))
console.log('JSON.stringify():', JSON.stringify(obj))
console.log('console.log:')
console.log(obj)
console.log(obj.Name)

这是chromium上编译版本的结果

Answer 8

XMLHttpRequest 实例上的所有属性都是继承属性，我的意思是它们不存在于实例本身，而是存在于它的原型链上。

这解释了为什么

const x = new XMLHttpRequest()
// all these methods consider only instance properties (owned properties) and thats why they return empty results
console.log(Object.keys(x)) // []
console.log(Object.getOwnPropertyNames(x)) // []
console.log(Object.entries(x)) // []
console.log(JSON.stringify(x)) // {}


// however if you try this you will get the results you want

console.log(Object.keys(x.__proto__)) // will not return symbol keys
console.log(Object.getOwnPropertyDescriptors(x.__proto__)) // will return descriptiors of every property whether its symbol or not

// you can see these results also by logging the xmlRequest, and looking at its __proto__ property in the console

// i will be calling the XMLHttpRequest instance for short as xmlRequest

您可以像这样简单地创建这样一个对象

class A {}
A.prototype.someProperty = 10
const a = new A()
// again if you try these you will get results like the XMLHttpRequest instance
console.log(Object.keys(a)) // []
console.log(Object.getOwnPropertyNames(a)) // []
console.log(Object.entries(a)) // []
console.log(JSON.stringify(a)) // {}

然而，当您尝试 console.log(a) 时，您将不会像使用 xmlRequest 实例那样看到 someProperty，这是预期的，因为它是继承的属性。您在记录 xmlRequest 时看到这些属性的原因是该对象正在接受特殊处理。

幸运的是，您可以使用 chrome custom object formatters 重新创建这种特殊处理，为此您必须从 chrome 开发人员工具设置

启用自定义格式化程序

window.devtoolsFormatters = [{
    header: function(obj){
        if (!(obj instanceof A)){
         return null;
        }
        // to get the exact results like the xmlRequest, you need to remove quotes from the object keys in the json string, can be done with a regex
        return ["div",{}, JSON.stringify(obj.__proto__)]
    },
    hasBody: function(){
        return false;
    }
}]

class A {}
A.prototype.someProperty = 10

console.log(new A()) // {"someProperty":10}

Answer 9

How can I create such an object in javascript, whose properties are only printed using console.log(obj), but not returned by any of the above functions?

一个接近的解决方案 我找到的是下面。这将打印带有 console.log(obj) 的属性。但是 obj 是一个代理。

在 chrome 控制台上，结果与 XMLHttpRequest 输出相同（class 为 Proxy）。在 firefox 控制台上，它表示 Proxy class，因此它并不完全相同。


    const obj = new Proxy({prop1:"value1", prop2: "value2"}, {ownKeys: _=>[]});

    console.log("Object.keys():",Object.keys(obj));
    console.log("Object.getOwnPropertyNames():",Object.getOwnPropertyNames(obj))
    console.log("Object.entries():",Object.entries(obj))
    console.log("JSON.stringify():",JSON.stringify(obj))
    console.log(obj)

结果：

Object.keys(): []
Object.getOwnPropertyNames(): []
Object.entries(): []
JSON.stringify(): {}
console.log:
Proxy {prop1: "value1", prop2: "value2"}

和console.log(obj.prop1) / obj.prop1="newValue"仍然有效。

Answer 10

这应该可以解决问题。

var obj = Object.create({
    get name(){
        return 'I go only to console.log()';
    }
});
console.log("Object.keys():",Object.keys(obj));
console.log("Object.getOwnPropertyNames():",Object.getOwnPropertyNames(obj));
console.log("Object.entries():",Object.entries(obj));
console.log("JSON.stringify():",JSON.stringify(obj));

console.log("console.log:", obj);

为什么 XMLHttpRequest 对象的属性只能通过 console.log() 打印？

Why properties of an XMLHttpRequest object are only printable through console.log()?

javascript

prototype

xmlhttprequest

prototypal-inheritance

ecmascript-6

创建您自己的此类对象