I think a better change would be to have this API return ReadonlyArray<...>.

Thanks for your review.
I agree it's better to return ReadonlyArray, in that case we freeze these arrays for manipulations.

Test are failing with error

Error: Symbol not found for identifier: readonly

I don't have any idea what does it mean.
Could you please give me a piece of advise?

The issue on here: https://github.com/angular/angular/pull/30397/files#diff-4a042642e9a286cfca1ed035a1fca39aR234
you're using readonly instead of ReadonlyArray

here as well: https://github.com/angular/angular/pull/30397/files#diff-4a042642e9a286cfca1ed035a1fca39aR297

@artem-galas - would you mind rebasing this on master?

Can you also tighten up the commit message a bit so that readers of the changelog will understand more what has changed. Perhaps something more like:

fix(common): mark locale data arrays as readonly

To discourage developers from mutating the arrays returned
from the following methods, their return types have been marked
as readonly.

* `getLocaleDayPeriods()`
* `getLocaleDayNames()`
* `getLocaleMonthNames()`
* `getLocaleEraNames()`

Fixes #27003

BREAKING CHANGE:
The locale data API has been marked as returning readonly arrays, rather
than mutable arrays, since these arrays are shared across calls to the
API. If you were mutating them (e.g. calling `sort()`, `push()`, `splice()`, etc)
then your code will not longer compile. If you need to mutate the array, you
should now take a copy (e.g. by calling `slice()`) and mutate the copy.

I just realised that this will probably fail due to the public API change. We will need to update the public API "golden files"...
The CI test failures will say what commands need to be run to do this.

I think it is worth running a presubmit now to see how many Google projects would be broken by this change.

Note to Caretakers: since this is a breaking change, we'd need to run Global presubmit as well once master branch is available for v11 changes.

Hi @artem-galas, the 10.1.0 version was released last week and master branch is now available for changes that target next major release (v11). Could you please rebase this PR, so that we re-run tests with the current version of master branch? Please let us know (in this thread) once this PR is rebased, so that we re-run tests in Google's codebase. Thank you.

Branch is rebased to latest master.

Presubmit + Global Presubmit.

FYI, presubmits (including a global one) went well, this PR is ready for merge.

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