Route Day.js operations through injected factory

Author: joshwooding

.changeset/dayjs-injected-factory.md

@@ -0,0 +1,5 @@
+---
+"@salt-ds/date-adapters": patch
+---
+
+Fixed the Day.js adapter to consistently use an injected Day.js factory for timezone resolution, UTC/timezone date creation, `today`, and `now`.

packages/date-adapters/src/__tests__/dayjs.spec.ts

@@ -1,4 +1,5 @@
 import dayjs from "dayjs";
+import "dayjs/locale/fr";
 import customParseFormat from "dayjs/plugin/customParseFormat";
 import timezone from "dayjs/plugin/timezone";
 import utc from "dayjs/plugin/utc";
@@ -11,9 +12,78 @@ dayjs.extend(utc);
 dayjs.extend(timezone);
 dayjs.extend(customParseFormat);
 
+const createInjectedDayjs = () => {
+  const fixedNow = "2030-06-15T12:34:56.000Z";
+  const systemTimezone = "Asia/Tokyo";
+  const injectedTimezone = ((
+    value?: Parameters<typeof dayjs.tz>[0],
+    timezone?: string,
+  ) => dayjs.tz(value ?? fixedNow, timezone)) as typeof dayjs.tz;
+  injectedTimezone.guess = () => systemTimezone;
+  injectedTimezone.setDefault = dayjs.tz.setDefault;
+  const injectedUtc = ((...args: Parameters<typeof dayjs.utc>) =>
+    args.length === 0
+      ? dayjs.utc(fixedNow)
+      : dayjs.utc(...args)) as typeof dayjs.utc;
+  const factory = Object.assign(
+    (...args: Parameters<typeof dayjs>) =>
+      args.length === 0 ? dayjs(fixedNow) : dayjs(...args),
+    { tz: injectedTimezone, utc: injectedUtc },
+  );
+
+  return { factory, fixedNow, systemTimezone };
+};
+
 describe("GIVEN a AdapterDayjs", () => {
   const adapter = new AdapterDayjs({ locale: "en" });
 
+  describe("GIVEN an injected Day.js factory", () => {
+    const { factory, fixedNow, systemTimezone } = createInjectedDayjs();
+    const injectedAdapter = new AdapterDayjs({ locale: "fr" }, factory);
+
+    it.each([
+      "default",
+      "system",
+    ] as const)("SHOULD use its timezone configuration for %s dates", (timezone) => {
+      const date = injectedAdapter.date("2025-01-05T00:00:00", timezone);
+
+      expect(injectedAdapter.getTimezone(date)).toBe(systemTimezone);
+      expect(date.format("Z")).toBe("+09:00");
+    });
+
+    it("SHOULD use its UTC factory", () => {
+      const date = injectedAdapter.date("2025-01-05T00:00:00", "UTC");
+
+      expect(date.utcOffset()).toBe(0);
+    });
+
+    it("SHOULD use its timezone factory for an explicit IANA timezone", () => {
+      const date = injectedAdapter.date(
+        "2025-01-05T00:00:00",
+        "America/New_York",
+      );
+
+      expect(injectedAdapter.getTimezone(date)).toBe("America/New_York");
+      expect(date.format("Z")).toBe("-05:00");
+    });
+
+    it("SHOULD apply the configured locale", () => {
+      const date = injectedAdapter.date("2025-01-05T00:00:00", "UTC");
+
+      expect(injectedAdapter.format(date, "MMMM")).toBe("janvier");
+    });
+
+    it("SHOULD use its clock for today", () => {
+      expect(injectedAdapter.today("UTC").format("YYYY-MM-DD HH:mm:ss")).toBe(
+        "2030-06-15 00:00:00",
+      );
+    });
+
+    it("SHOULD use its clock for now", () => {
+      expect(injectedAdapter.now("UTC").toISOString()).toBe(fixedNow);
+    });
+  });
+
   it("SHOULD create a Dayjs date in the system timezone", () => {
     const date = adapter.date("2023-11-01", "system");
     expect(date.isValid()).toBe(true);

packages/date-adapters/src/dayjs-adapter/index.ts

@@ -16,7 +16,11 @@ import {
 
 type Constructor = {
   (...args: Parameters<typeof defaultDayjs>): Dayjs;
-  tz?: (value: Parameters<typeof defaultDayjs>[0], timezone: string) => Dayjs;
+  tz?: {
+    (value: Parameters<typeof defaultDayjs>[0], timezone: string): Dayjs;
+    guess?: () => string;
+    setDefault?: (timezone?: string) => void;
+  };
   utc?: (value?: Parameters<typeof defaultDayjs>[0]) => Dayjs;
 };
 
@@ -88,7 +92,7 @@ export class AdapterDayjs implements SaltDateAdapter<Dayjs, string> {
       return undefined;
     }
     if (timezone === "system") {
-      return defaultDayjs.tz.guess();
+      return this.dayjs.tz?.guess?.() ?? defaultDayjs.tz.guess();
     }
     return timezone;
   };
@@ -102,11 +106,11 @@ export class AdapterDayjs implements SaltDateAdapter<Dayjs, string> {
     if (!this.dayjs.tz) {
       throw new Error("Salt Day.js adapter: missing timezone plugin");
     }
-    const timezone = defaultDayjs.tz.guess();
+    const timezone = this.dayjs.tz.guess?.() ?? defaultDayjs.tz.guess();
     if (timezone !== "UTC") {
-      return defaultDayjs.tz(value, timezone);
+      return this.dayjs.tz(value, timezone);
     }
-    return defaultDayjs(value);
+    return this.dayjs(value);
   };
 
   /**