fix: return dates as numbers from customTransformOutput (#298)

Reverts the adapter change from #236 that returned Date objects instead
of numbers. Updates tests to expect numbers for date fields.

Fixes the Convex serialization error reported in #288. Convex doesn't
support the Date type, so date fields must be numbers. The original
change in #236 was attempting to fix a type mismatch (#224), which may
be an upstream better-auth issue (tracked in
better-auth/better-auth#8619).

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: erquhart

src/client/adapter.ts

@@ -200,7 +200,7 @@ export const convexAdapter = <
       },
       customTransformOutput: ({ data, fieldAttributes }) => {
         if (data && fieldAttributes.type === "date") {
-          return new Date(data);
+          return new Date(data).getTime();
         }
         return data;
       },

src/test/adapter-factory/auth-flow.ts

@@ -82,8 +82,8 @@ export const getAuthFlowSuiteTests = (
 			user: User;
 			session: Session;
 		};
-		signUpResult.user.createdAt = new Date(signUpResult.user.createdAt);
-		signUpResult.user.updatedAt = new Date(signUpResult.user.updatedAt);
+		signUpResult.user.createdAt = new Date(signUpResult.user.createdAt).getTime() as unknown as Date;
+		signUpResult.user.updatedAt = new Date(signUpResult.user.updatedAt).getTime() as unknown as Date;
 		expect(result?.user).toBeDefined();
 		expect(result?.user).toStrictEqual(signUpResult.user);
 		expect(result?.session).toBeDefined();
@@ -117,8 +117,8 @@ export const getAuthFlowSuiteTests = (
 			body: { email: user.email, password: password },
 		});
 		process.env.TZ = "America/Los_Angeles";
-		expect(userSignUp.user.createdAt.toISOString()).toStrictEqual(
-			userSignIn.user.createdAt.toISOString(),
+		expect(userSignUp.user.createdAt).toStrictEqual(
+			userSignIn.user.createdAt,
 		);
 	},
 	"should sign up with additional fields": async () => {
@@ -135,7 +135,7 @@ export const getAuthFlowSuiteTests = (
 				name: user.name,
 				password: crypto.randomUUID(),
 				//@ts-expect-error - we are testing with additional fields
-				dateField: dateField.toISOString(), // using iso string to simulate client to server communication (this should be converted back to Date)
+				dateField: dateField.toISOString(), // using iso string to simulate client to server communication
 			},
 			asResponse: true,
 		});
@@ -145,7 +145,7 @@ export const getAuthFlowSuiteTests = (
 			headers,
 		});
 		//@ts-expect-error - we are testing with additional fields
-		expect(result?.user.dateField).toStrictEqual(dateField);
+		expect(result?.user.dateField).toStrictEqual(dateField.getTime());
 	},
 });
 

src/test/adapter-factory/basic.ts

@@ -68,7 +68,11 @@ export const getNormalTestSuiteTests = (
 			const transformed = transformGeneratedModel(user);
 			// console.log(`transformed:`, transformed);
 			// console.log(`result:`, result);
-			expect(result).toEqual(transformed);
+			expect(result).toEqual({
+				...transformed,
+				createdAt: new Date(user.createdAt).getTime(),
+				updatedAt: new Date(user.updatedAt).getTime(),
+			});
 		},
 		"create - should always return an id": async () => {
 			const { id: _, ...user } = await generate("user");
@@ -599,7 +603,7 @@ export const getNormalTestSuiteTests = (
 				where: [{ field: "createdAt", value: user.createdAt, operator: "eq" }],
 			});
 			expect(result).toEqual(user);
-			expect(result?.createdAt).toBeInstanceOf(Date);
+			expect(typeof result?.createdAt).toBe("number");
 			expect(result?.createdAt).toEqual(user.createdAt);
 		},
 		"findOne - should perform backwards joins": async () => {
@@ -753,7 +757,7 @@ export const getNormalTestSuiteTests = (
 				});
 				if (result?.session?.length) {
 					result.session = result.session.sort(
-						(a, b) => a.createdAt.getTime() - b.createdAt.getTime(),
+						(a, b) => (a.createdAt as unknown as number) - (b.createdAt as unknown as number),
 					);
 				}
 
@@ -869,7 +873,7 @@ export const getNormalTestSuiteTests = (
 		"findMany - should find many models with date fields": async () => {
 			const users = (await insertRandom("user", 3)).map((x) => x[0]);
 			const youngestUser = users.sort(
-				(a, b) => b.createdAt.getTime() - a.createdAt.getTime(),
+				(a, b) => (b.createdAt as unknown as number) - (a.createdAt as unknown as number),
 			)[0]!;
 			const result = await adapter.findMany<User>({
 				model: "user",
@@ -1709,7 +1713,7 @@ export const getNormalTestSuiteTests = (
 		"findMany - should find many models with gt operator": async () => {
 			const users = (await insertRandom("user", 3)).map((x) => x[0]);
 			const oldestUser = users.sort(
-				(a, b) => a.createdAt.getTime() - b.createdAt.getTime(),
+				(a, b) => (a.createdAt as unknown as number) - (b.createdAt as unknown as number),
 			)[0]!;
 			const result = await adapter.findMany<User>({
 				model: "user",
@@ -1730,7 +1734,7 @@ export const getNormalTestSuiteTests = (
 		"findMany - should find many models with gte operator": async () => {
 			const users = (await insertRandom("user", 3)).map((x) => x[0]);
 			const oldestUser = users.sort(
-				(a, b) => b.createdAt.getTime() - a.createdAt.getTime(),
+				(a, b) => (b.createdAt as unknown as number) - (a.createdAt as unknown as number),
 			)[0]!;
 			const result = await adapter.findMany<User>({
 				model: "user",

src/test/adapter-factory/convex-custom.ts

@@ -757,7 +757,7 @@ export const convexCustomTestSuite = createTestSuite(
           where: [{ field: "createdAt", value: now }],
         }),
       ).toEqual(user);
-      expect(user.createdAt).toBeInstanceOf(Date);
+      expect(typeof user.createdAt).toBe("number");
     },
   }),
 );