JavaScript-计算属性-深层困惑 [英] JavaScript - Computed properties - deep confusion

问题描述

似乎我对JavaScript中的计算属性感到很困惑.

Seems I am quite confused by computed properties in JavaScript.

当我定义一个对象并将[d]用作键(作为属性键/名称)时，该[d]实际做什么?似乎对于某些值d，它会计算s = d.toString()并将该值s用作属性键. 但是对于其他值d(例如，当d是符号时)，它实际上使用该符号的值作为键.

When I define an object and I put [d] as a key (as a property key/name) what does this [d] actually do? Seems that for some values d it calculates s = d.toString() and uses that value s as the property key. But for other values d (e.g. when d is a symbol) it uses really the symbol's value as the key.

因此，[d]的这种双重行为(作为语法构造)似乎令人困惑.有人可以深入解释它是如何工作的吗?

So this dual behavior of [d] (as a syntax construct) seems confusing. Could someone explain in depth how this works?

还有其他特殊情况吗?还是只有d是Symbol时，我们才有这种特殊行为?

Are there other special cases btw? Or is it just when d is a Symbol when we have that special behavior?

回到基础知识:什么事物可以是对象属性的键/名称?它是只是字符串还是只是字符串和符号，还是还有其他附加内容...?

Back to the basics: what things can be keys/names of properties of an object? Is it just strings or just strings and symbols or is there also something additional... ?

示例:

var symbol = Symbol("test");

function Animal(name){
	this.name = name;
}

Animal.prototype = {};
Animal.prototype.constructor = Animal;

function Dog(breed){
    this.breed = breed;
    this.name = "Dog";
    this.s = symbol;
}

Dog.prototype = new Animal();
Dog.prototype.constructor = Dog;

console.log("001");
var d = new Dog("Sharo");
for (let x in d){
    console.log(x, ":", d[x]);
}

console.log("002");
d = new Object();
for (let x in d){
    console.log(x, ":", d[x]);
}

console.log("003");
d = new Number(5);
for (let x in d){
    console.log(x, ":", d[x]);
}

var d1 = {};
var d2 = {};

var d = new Dog("Sharo");

var m = {[d1] : 5, [d2] : 10, [d] : 20, z : 100, symbol: 2000, [symbol] : 5000};

console.log("============================");
console.log(m);

for (let x in m){
    console.log(x, ":", m[x]);
}
console.log("============================");

推荐答案

由于似乎没有人愿意回答这个问题，我将根据上面的评论自己回答这个问题，由于这个原因，我不再感到困惑.

Since no one seems interested in answering this question I will answer it myself based on the comments which I got above, and due to which I am not confused anymore.

请注意，此处的答案基于 ES6 .我的意思是……谁知道JavaScript的未来还有什么呢:)

Note that this answer here is ES6 based. I mean... who knows what else the JavaScript future will hold :)

当我定义一个对象并将[d]用作键(作为属性键/名称)时，该[d]实际做什么?似乎对于某些对象d，它会计算s = d.toString()并将该值s用作属性键.但是对于其他对象d(例如，当d是Symbol时)，它实际上使用Symbol的值作为键.

When I define an object and I put [d] as a key (as a property key/name) what does this [d] actually do? Seems that for some objects d it calculates s = d.toString() and uses that value s as the property key. But for other objects d (e.g. when d is a Symbol) it uses really the Symbol's value as the key.

是的，这是正确的.当d为Symbol时，其值将直接使用.当d是Symbol以外的任何值时，其值将强制为字符串，并且该字符串用作属性名称/键.强制更像是String(d)而不是d.toString().

Yes, that's correct. When d is a Symbol its value is used directly. When d is anything but Symbol its value is coerced to a string and that string is used as the property name/key. The coercion is more like String(d) rather than d.toString().

因此，[d]的这种双重行为(作为语法构造)似乎令人困惑.有人可以深入解释它是如何工作的吗?

So this dual behavior of [d] (as a syntax construct) seems confusing. Could someone explain in depth how this works?

已经在上面进行了解释.

Already explained above.

还有其他特殊情况吗?还是只有d是Symbol时，我们才有这种特殊行为?

Are there other special cases btw? Or is it just when d is a Symbol when we have that special behavior?

没有其他特殊情况".从ES6开始，只有字符串和符号可以是属性键.

There are no other "special cases". As of ES6 only strings and symbols can be property keys.

回到基础知识:什么事物可以是对象属性的键/名称?它是只是字符串还是只是字符串和符号，还是还有其他附加内容...?

Back to the basics: what things can be keys/names of properties of an object? Is it just strings or just strings and symbols or is there also something additional... ?

正如已经说过的那样，从ES6开始，只有字符串和符号可以是属性键.

As already said, as of ES6 only strings and symbols can be property keys.

参考:

(1) https://developer.mozilla.org/zh-美国/docs/Web/JavaScript/Reference/Operators/Property_Accessors

属性名称是字符串或符号.任何其他值(包括数字)都被强制为字符串."

"Property names are string or Symbol. Any other value, including a number, is coerced to a string."

(2) https://www.ecma-international.org/ecma-262/6.0/#sec-propertykey

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