为什么可以通过Object.defineProperty枚举ES6符号属性？ [英] Why can ES6 Symbol properties be made enumerable by Object.defineProperty?

问题描述

  var symbol = Symbol（）; 
 var object = {}; 
 object [symbol] ='value'; 
  

MDN将枚举属性定义为可由for..in循环迭代的属性（1 ）。符号属性永远不会被一个for ... in循环迭代，因此它们可以被认为是非枚举的（2）。

这样做是否有意义呢？你可以这样做：

  Object.defineProperty（object，symbol，{
 value：'value'，
可枚举：true 
}）; 
  

该查询对象为其描述符确实确认此属性是可枚举的：

  Object.getOwnPropertyDescriptor（object，symbol）
 //  - > {enumerable：true} 
  

为什么？这是什么用？

（1） https://developer.mozilla.org/en-US/docs/Web/JavaScript/Enumerability_and_ownership_of_properties

（2）for ... in ... [ [枚举]] ，其中只包含字符串键。可能，现在我们应该改变MDN上的定义，我们有符号属性。

解决方案

是的，有一个允许符号属性可枚举： Object.assign ：

  let s1 = Symbol（）; 
 let s2 = Symbol（）; 
 let s3 = Symbol（）; 
 let original = {}; 
 original [s1] =value1; // Enumerable 
 Object.defineProperty（original，s2，{// Enumerable 
 enumerable：true，
 value：value2
}）; 
 Object.defineProperty（original，s3，{// Non-enumerable 
 value：value3
}）; 
 let copy = {}; 
 Object.assign（copy，original）; 
 console.log（copy [s1] is+ copy [s1]）; // value1，因为它是可枚举的
 console.log（copy [s2] is+ copy [s2]）; // value2，因为它是可枚举的
 console.log（copy [s3] is+ copy [s3]）; //未定义，因为它不是可枚举的
  

Live Copy 。

为了清楚起见：

MDN将枚举属性定义为可由for..in循环迭代的属性（1）。

对于ES6（ES2015）来说这是错误的。在ES5和更早版本中，这是一个合理的，如果是简单化的定义，不会因为 Symbol 而不再简单地更正。我已经修复了这篇文章。

---

这是一个CW，因为它是对这个问题的评论的产物。 / p>

In ES6 properties can be defined as symbol properties:

var symbol = Symbol();
var object = {};
object[symbol] = 'value';

MDN defines enumerable properties as 'those which can be iterated by a for..in loop' (1). Symbol properties are never iterated by a for...in loop, therefore they can be considered non-enumerable (2).

Does it make any sense, then, that you can do this:

Object.defineProperty(object, symbol, {
    value: 'value',
    enumerable: true
});

and that querying object for it's descriptor does indeed confirm that this property is enumerable:

Object.getOwnPropertyDescriptor(object, symbol)
// -> { enumerable: true }

Why? What use is this?

(1) https://developer.mozilla.org/en-US/docs/Web/JavaScript/Enumerability_and_ownership_of_properties

(2) for...in uses [[Enumerate]], which only includes string keys. Probably the definition on MDN should be changed now that we have symbol properties.

解决方案

Yes, there's a reason for allowing Symbol properties to be enumerable: Object.assign:

let s1 = Symbol();
let s2 = Symbol();
let s3 = Symbol();
let original = {};
original[s1] = "value1";                // Enumerable
Object.defineProperty(original, s2, {   // Enumerable
  enumerable: true,
  value: "value2"
});
Object.defineProperty(original, s3, {   // Non-enumerable
  value: "value3"
});
let copy = {};
Object.assign(copy, original);
console.log("copy[s1] is " + copy[s1]); // value1, because it was enumerable
console.log("copy[s2] is " + copy[s2]); // value2, because it was enumerable
console.log("copy[s3] is " + copy[s3]); // undefined, because it wasn't enumerable

Live Copy on Babel's REPL.

Just for clarity:

MDN defines enumerable properties as 'those which can be iterated by a for..in loop' (1).

That's simply wrong for ES6 (ES2015). It was a reasonable, if simplistic, definition in ES5 and earlier, no it's no longer even simplistically correct because of Symbols. I've fixed the article.

---

This is a CW because it was the outgrowth of the comments on the question.

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