Ignore non-standard array keys

Question

Say we have this:

const v = [1,2,3];
v.foo = 5;

console.log(JSON.stringify(v));

we will get:

'[1,2,3]'

and the 'foo' property on the array will be ignored. However, if we do:

console.log(Object.keys(v));

we will get:

[ '0', '1', '2', 'foo' ]

my question is - how can we get only the standard keys from an array, and ignore any properties that were tacked onto the array?

The only way I know how to do this would be something like:

const keys = Object.keys(v).filter(k => Number.isInteger(parseInt(k)));

is there a better way?

Why would you add a key to an array? How do you even end up in such a situation, that you need to fix that? — Kresimir, Dec 08 '18 at 22:17
@Kresimir it's for a library, I can't control what other people do. — Alexander Mills, Dec 08 '18 at 22:17
@kinduser the library needs to stringify an object, but handle circular references. JSON.stringify will throw an error upon circular refs. — Alexander Mills, Dec 08 '18 at 22:19

Bergi · Accepted Answer · 2018-12-08T22:36:35.933

5

You can use

Array.from(v.keys())

where keys returns an iterator over integer indices (not property names!). Notice that in contrast to Object.keys it ignores the sparsity of arrays and produces an index even for holes.

I wonder though why you even have to use Object.keys on an array. An array has indices only, and you can generate them with a simple for (let i=0; i<v.length; i++) loop. Or even with something like Array.from({length: v.length}, (_, i) => i).

edited Dec 08 '18 at 22:36

answered Dec 08 '18 at 22:25

Bergi

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yep this is right, I am using `for(let k of keys)` so just `v.keys()` is good. – Alexander Mills Dec 08 '18 at 22:29

score 1 · Answer 2 · answered Dec 08 '18 at 22:35

It is advised to not mess with array keys. V8 does performance evaluation based on array keys and does some pretty nice assumptions which if you modify you will loose all the benefits.

But in case you don't care about performance, than you can create a copy of the array and the engine wont copy custom keys.

Object.keys([...v])

score 0 · Answer 3 · answered Dec 08 '18 at 22:17

0

You can use Array.from.

const v = [1,2,3];
v.foo = 5;
const keys = Array.from(v);
console.log(keys);

answered Dec 08 '18 at 22:17

eag845

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I am looking for the keys of the array, not the values. I think I am looking for `Array.prototype.keys()` => https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/keys – Alexander Mills Dec 08 '18 at 22:19

Ignore non-standard array keys

3 Answers3