Why does accessing an element in an object using an array as a key work?

Question

What do you make of this?

var x = {a: 1};         //=> {a: 1}
var y = Object.keys(x); //=> [\'a\']
x[y]                    //=> 1

Answer 1

I think it come from 2 facts :

1. The JS engine type as it goes
2. any JS object can be access "like" an array

so your

["a"] 

when used as a key will be "casted" to string, which give

a

Even

[[[[["a"]]]]].toString() == a // true

so your

x[y] 

end un as

x["a"]

Answer 2

@Quentin suggests that property names are automatically converted to strings. Ok, I think he's onto something there, but giving two explicit arr.toString() examples doesn't really demonstrate accessing the property of an object using an array. I offered this as an edit to his post. However, he rolled it back.

Anyway, this demonstrates the implicit behavior much more evidently, imo.

var x = {'a,b,c': 1};

var y = ['a','b','c'];

x[y]; //=> 1

Answer 3

Property names have to be strings. If you try to use an array as a property name, it gets its toString() method called implicitly. That generates a string containing a comma-separated list of its values.

> var array = ['a', 'b', 'c'];
undefined
> array.toString();
'a,b,c'

If you only have one value, then there aren't any commas.

> var array = ['a'];
undefined
> array.toString();
'a'