Intl Era Monthcode: Test DateTimeFormat era formatting

Tests how Intl.DateTimeFormat formats eras in various calendars. The
actual output is implementation-defined, but we can test a few things:

- Calendars with multiple eras have distinct values for the era part
  when formatting dates in each era.
- Calendars with one era have the same value for the era part when
  formatting dates with positive and negative era years.
- Calendars without eras do not include an era part in the format.

Some calendars are missing from this test because their era
configurations were updated in CLDR 48. I will add those in a separate
PR which can be merged when implementations have had more time to update
to that version of CLDR.

Co-Authored-By: Philip Chimento <pchimento@igalia.com>

test/intl402/DateTimeFormat/prototype/formatToParts/era.js

@@ -0,0 +1,105 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-intl.datetimeformat.prototype.formattoparts
+description: >
+  Verifies that DateTimeFormat formats era information, with distinct values for
+  calendars with multiple eras
+locale: [en]
+features: [Intl.Era-monthcode]
+---*/
+
+const multipleEraTests = [
+  ["gregory", [-100, 2025]],
+  ["japanese", [-100, 1850, 1880, 1920, 1930, 1990, 2025]],
+  ["roc", [1900, 2025]],
+];
+
+for (const [calendar, isoYears] of multipleEraTests) {
+  const formatter = new Intl.DateTimeFormat("en", {
+    calendar,
+    era: "long",
+    year: "numeric",
+  });
+
+  const eras = [];
+
+  for (const isoYear of isoYears) {
+    const date = new Date(isoYear, 5, 15);
+    const parts = formatter.formatToParts(date);
+
+    const eraPart = parts.find(({ type }) => type === "era");
+    assert.notSameValue(eraPart, undefined, `Format of ${calendar} ISO ${isoYear} should have era part`);
+    assert.sameValue(typeof eraPart.value, "string", `Era format of ${calendar} ISO ${isoYear} should be a string`);
+    assert(eraPart.value.length > 0, `Era format of ${calendar} ISO ${isoYear} should not be empty`);
+    eras.push(eraPart.value);
+
+    const format = formatter.format(date);
+    assert(format.includes(eraPart.value), `${format} should include ${eraPart.value} era`);
+
+    const yearPart = parts.find(({ type }) => type === "year");
+    assert.notSameValue(yearPart, undefined, `Format of ${calendar} ISO ${isoYear} should have year part`);
+  }
+
+  assert.sameValue(new Set(eras).size, eras.length, `${calendar} eras (${eras.join(",")}) should be unique`);
+}
+
+// Calendars with a single era: test one year occurring before and one after the
+// single era's epoch
+const singleEraTests = [
+  ["buddhist", [-600, 2025]],
+  ["ethioaa", [-5550, 2025]],
+  ["hebrew", [-3800, 2025]],
+  ["indian", [70, 2025]],
+  ["persian", [600, 2025]],
+];
+
+for (const [calendar, isoYears] of singleEraTests) {
+  const formatter = new Intl.DateTimeFormat("en", {
+    calendar,
+    era: "long",
+    year: "numeric",
+  });
+
+  const eras = [];
+
+  for (const isoYear of isoYears) {
+    const date = new Date(isoYear, 5, 15);
+    const parts = formatter.formatToParts(date);
+
+    const eraPart = parts.find(({ type }) => type === "era");
+    assert.notSameValue(eraPart, undefined, `Format of ${calendar} ISO ${isoYear} should have era part`);
+    assert.sameValue(typeof eraPart.value, "string", `Era format of ${calendar} ISO ${isoYear} should be a string`);
+    assert(eraPart.value.length > 0, `Era format of ${calendar} ISO ${isoYear} should not be empty`);
+    eras.push(eraPart.value);
+
+    const format = formatter.format(date);
+    assert(format.includes(eraPart.value), `${format} should include ${eraPart.value} era`);
+
+    const yearPart = parts.find(({ type }) => type === "year");
+    assert.notSameValue(yearPart, undefined, `Format of ${calendar} ISO ${isoYear} should have year part`);
+  }
+
+  assert.sameValue(eras[0], eras[1], `${calendar} era does not change between negative to positive years`);
+}
+
+const noEraTests = [
+  "chinese",
+  "dangi",
+  "iso8601",
+];
+
+for (const calendar of noEraTests) {
+  const formatter = new Intl.DateTimeFormat("en", {
+    calendar,
+    era: "long",
+    year: "numeric",
+  });
+
+  const date = new Date(2025, 5, 15);
+  const parts = formatter.formatToParts(date);
+
+  const eraPart = parts.find(({ type }) => type === "era");
+  assert.sameValue(eraPart, undefined, `Format of ${calendar} should not have era part`);
+}