feat(intl): implement Temporal.PlainYearMonth.prototype.toLocaleString by alienx5499 · Pull Request #5098 · boa-dev/boa

alienx5499 · 2026-03-15T11:06:47Z

This Pull Request fixes/closes #5083.

It changes the following:

Implement Temporal.PlainYearMonth.prototype.toLocaleString per spec. It now acts as a thin wrapper that delegates to a new, centralized HandleDateTimeValue dispatcher (§15.6.22) in the Intl module. When the caller omits dateStyle/timeStyle, default year: "numeric" and month: "short" are applied so ICU does not fall back to full date defaults (reference day) — fixes Test262 default-does-not-include-day-time-and-time-zone-name.js.
Implement HandleDateTimeTemporalYearMonth (§15.6.16) to compute the Intl formatting anchor from the Temporal plain value; validate Intl calendar against the Temporal calendar, and model Temporal plain timeZone by forcing Intl timeZone to "+00:00".
Add tests for PlainYearMonth.prototype.toLocaleString: returns string, invalid receiver throws TypeError, and (with the intl feature) different locales/options affect output; plain timeZone is ignored/overridden, incompatible calendar throws, and default output excludes reference day/time/timeZoneName (regression for Test262 default-does-not-include-day-time-and-time-zone-name.js).

Testing

cargo test -p boa_engine --no-default-features --features temporal --lib plain_year_month (Temporal-only tests)
cargo test -p boa_engine --features intl_bundled,temporal --lib plain_year_month (with intl)
cargo run -p boa_tester -- run --suite test/intl402/Temporal/PlainYearMonth/prototype/toLocaleString/default-does-not-include-day-time-and-time-zone-name.js

Status: rebased onto main after #5080; this PR now only contains the Temporal PlainYearMonth.prototype.toLocaleString changes for #5083.

github-actions · 2026-03-15T11:16:01Z

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	50,079	50,080	+1
Ignored	2,072	2,072	0
Failed	812	811	-1
Panics	0	0	0
Conformance	94.55%	94.56%	+0.00%

Fixed tests (1):

test/intl402/Temporal/PlainYearMonth/prototype/toLocaleString/datestyle-and-timestyle.js (previously Failed)

Tested main commit: 26271b7c3f6f357f12fd835a0402d8b643f03db5
Tested PR commit: fceb44a4e37de47b171fe2b919a45e4fef706734
Compare commits: 26271b7...fceb44a

codecov · 2026-03-15T11:33:38Z

Codecov Report

❌ Patch coverage is 76.34409% with 22 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.97%. Comparing base (6ddc2b4) to head (a1db048).
⚠️ Report is 951 commits behind head on main.

Files with missing lines	Patch %	Lines
...e/engine/src/builtins/intl/date_time_format/mod.rs	75.00%	22 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5098       +/-   ##
===========================================
+ Coverage   47.24%   59.97%   +12.72%     
===========================================
  Files         476      590      +114     
  Lines       46892    63766    +16874     
===========================================
+ Hits        22154    38243    +16089     
- Misses      24738    25523      +785

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nekevss

Generally worth noting, that from what I can tell. The T.p.toLocaleString implementations all seem to require similar plumbing.

The specification can be found here

nekevss · 2026-03-20T09:25:22Z

-        Ok(JsString::from(year_month.inner.to_string()).into())
+        #[cfg(feature = "intl")]
+        {
+            use crate::builtins::{


issue: the implementation here should ideally follow or adapt the specification steps provided in the Temporal specification as additions to ECMA402

…h ECMA-402

github-actions · 2026-03-20T18:12:04Z

Test262 conformance changes

Test result	main count	PR count	difference
Total	53,125	53,125	0
Passed	51,049	51,052	+3
Ignored	1,482	1,482	0
Failed	594	591	-3
Panics	0	0	0
Conformance	96.09%	96.10%	+0.01%

Fixed tests (3):

test/intl402/Temporal/PlainYearMonth/prototype/toLocaleString/lone-options-accepted.js (previously Failed)
test/intl402/Temporal/PlainYearMonth/prototype/toLocaleString/datestyle-and-timestyle.js (previously Failed)
test/intl402/Temporal/PlainYearMonth/prototype/toLocaleString/options-conflict.js (previously Failed)

Tested main commit: 3cec5e6bc4454077cb8d65b537d368d633137e0e
Tested PR commit: a1db0486e9b3ea6b9726ec0bef8508e42005ca5c
Compare commits: 3cec5e6...a1db048

alienx5499 · 2026-03-22T18:28:20Z

Hi @nekevss,

I’ve addressed your feedback and aligned the implementation more closely with the Temporal -> ECMA-402 specification steps, and rebased on latest main.

Since #5080 is now merged, the original blocking dependency should be resolved. Could you please take another look when you have time?

If the Blocked label was only due to that dependency, it may no longer be needed.

Let me know if anything still needs adjustment.

Thanks!

nekevss

This is looking good overall and the tests appear to be promising.

This method is a little tricky, especially when it comes to the specification. There's a lot of complicated specification text, but in order to better support the other temporal built-ins and follow up work on this code path.

It's worth noting that if there's other code paths that you aren't trying to support currently. Feel free to return a result where the other branches return a js_error!("Not yet implemented").

nekevss · 2026-03-25T03:57:38Z

+            }
+
+            // Compute epochNs from isoDate + NoonTimeRecord (steps 2-3), then pass it to the shared
+            // ECMA-402 formatting pipeline as epoch milliseconds.


issue: this should ideally not be inlined from HandleDateTimeValue.

The implementation here should probably stick as close to the specification as possible while also being able to expand to support the other temporal built-ins

…ormatting

alienx5499 · 2026-03-25T10:04:28Z

Hi @nekevss,

Updated based on your feedback, moved the Temporal.PlainYearMonth logic out of HandleDateTimeValue and into a dedicated handler, keeping the dispatcher clean and closer to the spec structure.

Would appreciate a re-review when you have time.

Thanks!

alienx5499 · 2026-04-07T20:34:45Z

Hi @nekevss,

Just a quick followup, the earlier feedback has been addressed.

Happy to refine further if needed. Would appreciate a re review when you get a chance.

Thanks!

Vellumic · 2026-04-13T03:01:30Z

I noticed this PR uses logic that violates the ECMA-402 specification regarding dateStyle/timeStyle fallbacks. I've already submitted a separate bugfix and regression test for this issue here: #5324.

Furthermore, this PR duplicates parts of the formatting infrastructure I'm currently working on in #5199. We should avoid merging these changes to prevent double-work and immediate technical debt.

…ormatting

alienx5499 · 2026-04-13T08:31:26Z

@Vellumic
You’re right about the dateStyle/timeStyle fallback, that was introduced earlier in #5080 and is now corrected here, along with regression tests.

This PR focuses on implementing Temporal.PlainYearMonth.prototype.toLocaleString #5083 with minimal, spec-aligned changes.

Vellumic · 2026-04-14T01:32:06Z

I see you used my fix in your code, however it doesn't fixes the general issue I was addressing earlier. Because Temporal was designed with legacy Date flaws in mind, it needs more complex parsing.

alienx5499 · 2026-04-14T04:17:44Z

@Vellumic

The change here addresses the dateStyle/timeStyle fallback behavior in the shared Date formatting path, which this PR depends on.

This PR is scoped to Temporal.PlainYearMonth.prototype.toLocaleString #5083. Broader Temporal formatting concerns should be handled separately if needed.

github-actions bot added this to the v1.0.0 milestone Mar 15, 2026

alienx5499 mentioned this pull request Mar 15, 2026

Implement Temporal.PlainYearMonth.prototype.toLocaleString #5083

Open

This was referenced Mar 15, 2026

Implement Temporal.PlainTime.prototype.toLocaleString #5084

Open

Implement Temporal.PlainMonthDay.prototype.toLocaleString #5085

Open

Implement Temporal.PlainDateTime.prototype.toLocaleString #5086

Open

jedel1043 added the Blocked Waiting for another code change label Mar 16, 2026

feat(intl): implement Temporal.PlainYearMonth.prototype.toLocaleString

fceb44a

alienx5499 force-pushed the feat/5083-plainyearmonth-tolocalestring branch from f11323d to fceb44a Compare March 19, 2026 10:30

github-actions bot added the Waiting On Review Waiting on reviews from the maintainers label Mar 19, 2026

alienx5499 marked this pull request as ready for review March 19, 2026 10:36

alienx5499 requested review from a team, jasonwilliams, jedel1043 and nekevss as code owners March 19, 2026 10:36

nekevss requested changes Mar 20, 2026

View reviewed changes

alienx5499 and others added 2 commits March 20, 2026 23:26

fix(intl): align Temporal.PlainYearMonth.prototype.toLocaleString wit…

3f5d49e

…h ECMA-402

Merge branch 'main' into feat/5083-plainyearmonth-tolocalestring

2220871

alienx5499 and others added 2 commits March 21, 2026 12:50

fix(intl): avoid Date style defaults in PlainYearMonth.toLocaleString

e887556

Merge branch 'main' into feat/5083-plainyearmonth-tolocalestring

f911f96

alienx5499 requested a review from nekevss March 21, 2026 08:01

nekevss requested changes Mar 25, 2026

View reviewed changes

refactor(intl): extract HandleDateTimeValue dispatcher for Temporal f…

ea730d8

…ormatting

alienx5499 requested a review from nekevss March 25, 2026 10:04

Merge branch 'main' into feat/5083-plainyearmonth-tolocalestring

0b41c72

alienx5499 and others added 2 commits April 13, 2026 13:02

Merge branch 'main' into feat/5083-plainyearmonth-tolocalestring

c1b5bfb

fix(intl): stop forcing dateStyle/timeStyle defaults in Date locale f…

a1db048

…ormatting

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