forms: Support negative indexes in `FormArray`

Which @angular/* package(s) are relevant/releated to the feature request?

forms

Description

Nowadays attempting to retrieve a control in a negative index returns undefined (due to underlying usage of native Array). I would like FormArray to support relative indexing. Methods like at() and removeAt() could support negative integers as indexes, which count back from the last item in the array.

Proposed solution

Similar to the ES2021 Array.prototype.at(), support negative integers as indexes:

 // the following are equivalent:
formArray.at(-1)
formArray.at(formArray.length - 1)

Methods which should support relative indexing:

1. at()
2. removeAt()
3. insert()
4. setControl()

See proposed solution implementation in https://github.com/angular/angular/pull/44631

Alternatives considered

One could use formArray.at(formArray.length - 1) like in indices of native Arrays pre-ES2021, but this can be exhausting and causes longer lines of code.

[AndrewKushnir]

@amitbeck thanks for creating this feature request. According to the Feature Request process (documented here), this ticket reached 20 upvotes and we’ve discussed it with the Angular team.

Given the fact that:

• FormArray class represents an Array data structure and using negative indices is common in array methods (such as splice and at)
• This feature would have minimal impact on the bundle size (we would even be able to pass the index as is in case of at function)

We’d like to proceed with this feature request. I’ll reopen PR #44631 and provide some feedback there.

Thank you.

[oran-intsights]

Very Important!