为什么可以使用数组访问对象属性? [英] Why can I access object property with an array?

问题描述

有人可以解释以下代码的行为吗?

Can somebody explain the behaviour of the following code?

let obj = {a:1, b:2}
let i = ['a']
console.log(obj[i])
>> 1

为什么连一个数组都可以用来访问对象内部的属性?附带说明，这仅适用于长度为1的数组.我曾尝试对此进行研究，但据我所知，目前还没有文档解释为什么这应该起作用.

Why is it that even an array can be used to access a property inside an object? As a side note this only works with an array of length 1. I have tried researching this but there's no documentation as far as I know that explains why this should work.

推荐答案

属性名称始终是字符串或

Property names are always strings or symbols.

如果传递的内容不是字符串或符号，则会将其转换为字符串.

If you pass something which isn't a string or symbol, it gets converted to a string.

数组上默认的 toString()方法大致为:

The default toString() method on an array is roughly:

String.prototype.toString = function () { return this.join(","); }

因此 ['a'] 被转换为'a'.

请注意，这仅适用于长度为1的数组.

As a side note this only works with an array of length 1.

对于更长的数组，它可以正常工作.您只需要一个匹配值:

It works fine with arrays that are longer. You just need a matching value:

const o = {
    "a,b": "Hello"
}
const a = ["a", "b"];
console.log("" + a);
console.log(o[a]);

由于任何对象都可以转换为字符串，并且您可以自定义 toString 方法，因此您可以做一些奇怪的事情:

And since any object can be converted to a string, and you can customise the toString method, you can do really weird things:

const data = {
  "42": "Hello"
}

class Weird {
    constructor(x) {
        this.x = x;
    }
    toString() {
        return this.x + 40;
    }
}

const w = new Weird(2);
console.log(data[w]);

(请注意，做一些真正奇怪的事情通常是一个愚蠢的主意，这使得在两周后很难调试自己的代码).

(Note that doing really weird things is usually a stupid idea that makes it hard to debug your own code two weeks later).

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