Not possible to set numeric keys, even with multiple numbers like "5.1.1", on an object

[BennyAlex]

My keys have multiple numbers seperated by dot.

In this check:

the isNaN return false. We should only check for positive whole numbers here, all other numbers are not valid array indexes.

But I also wonder why its not allowed to set a numeric key in an object at all. Its not going against any JS rules. It should be possible.

[erikras-gilfoyle-agent]

Fixed!

This has been fixed in final-form PR #526:
final-form/final-form#526

The Problem

Keys like "5.1.1" were being incorrectly treated as array indexes because the code only checked isNaN(Number(key)), which returns false for decimal numbers.

The Solution

Created an isValidArrayIndex() helper that checks if a key is a valid array index:

• Must be parseable as a number
• Must be a whole integer
• Must be non-negative
• Must be in canonical form (prevents "01" from being treated as 1)

Impact

✅ Decimal keys like "5.1.1" now work as object keys
✅ Negative numbers like "-1" treated as object keys
✅ Padded numbers like "01" treated as object keys
✅ Valid integers ("0", "42") still work as array indexes

Fixes #1042