Why can I access object property with an array?

问题

Can somebody explain the behaviour of the following code?

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

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.

回答1:

Property names are always strings or symbols.

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

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

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

So ['a'] gets converted to 'a'.

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

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]);

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]);

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

回答2:

let obj = {a:1, b:2}

First you declare an object with 2 properties, a and b with values 1 and 2, respectively.

let i = ['a']

Then a variable i is declared, with it's value set to a string array with a single element, 'a'.

console.log(obj[i])

In this statement, the value of i is resolved to a string because the array only contains one element, as you mentioned. Due to this, 'a' is a valid property name for obj because all object properties are either strings, and if you pass something such as an array, it's converted to a string.

If you reference the variable icontaining the array, and it has multiple elements, it cannot resolve to a single property name without you being more explicit such as obj[i[0]]. If you only have one value, it'll resolves to that value.

obj['a'] is a valid property obj['a,b'] is not.

来源：https://stackoverflow.com/questions/55119963/why-can-i-access-object-property-with-an-array

标签

javascript

arrays

object