Stub out `useFakeTimers` and `useRealTimers`

4 files changed, 96 insertions, 12 deletions

diff --git a/packages/bun-types/bun-test.d.ts b/packages/bun-types/bun-test.d.ts
index 156585766..5182f7e93 100644
--- a/packages/bun-types/bun-test.d.ts
+++ b/packages/bun-types/bun-test.d.ts
@@ -29,6 +29,36 @@ declare module "bun:test" {
     <T extends AnyFunction>(Function: T): Mock<T>;
   };
 
+  /**
+   * Control the system time used by:
+   * - `Date.now()`
+   * - `new Date()`
+   * - `Intl.DateTimeFormat().format()`
+   *
+   * In the future, we may add support for more functions, but we haven't done that yet.
+   *
+   * @param now The time to set the system time to. If not provided, the system time will be reset.
+   * @returns `this`
+   * @since v0.6.13
+   *
+   * ## Set Date to a specific time
+   *
+   * ```js
+   * import { setSystemTime } from 'bun:test';
+   *
+   * setSystemTime(new Date('2020-01-01T00:00:00.000Z'));
+   * console.log(new Date().toISOString()); // 2020-01-01T00:00:00.000Z
+   * ```
+   * ## Reset Date to the current time
+   *
+   * ```js
+   * import { setSystemTime } from 'bun:test';
+   *
+   * setSystemTime();
+   * ```
+   */
+  export function setSystemTime(now?: Date | number): ThisType<void>;
+
   interface Jest {
     restoreAllMocks(): void;
     fn<T extends AnyFunction>(func?: T): Mock<T>;
diff --git a/src/bun.js/bindings/JSMockFunction.cpp b/src/bun.js/bindings/JSMockFunction.cpp
index 0e24e761c..3a84f0139 100644
--- a/src/bun.js/bindings/JSMockFunction.cpp
+++ b/src/bun.js/bindings/JSMockFunction.cpp
@@ -66,6 +66,18 @@ JSC_DECLARE_HOST_FUNCTION(jsMockFunctionMockRejectedValueOnce);
 JSC_DECLARE_HOST_FUNCTION(jsMockFunctionWithImplementationCleanup);
 JSC_DECLARE_HOST_FUNCTION(jsMockFunctionWithImplementation);
 
+// This is a stub. Exists so that the same code can be run in Jest
+extern "C" EncodedJSValue JSMock__jsUseFakeTimers(JSC::JSGlobalObject* globalObject, JSC::CallFrame* callFrame)
+{
+    return JSValue::encode(callFrame->thisValue());
+}
+
+extern "C" EncodedJSValue JSMock__jsUseRealTimers(JSC::JSGlobalObject* globalObject, JSC::CallFrame* callFrame)
+{
+    globalObject->overridenDateNow = -1;
+    return JSValue::encode(callFrame->thisValue());
+}
+
 extern "C" EncodedJSValue JSMock__jsNow(JSC::JSGlobalObject* globalObject, JSC::CallFrame* callFrame)
 {
     return JSValue::encode(jsNumber(globalObject->jsDateNow()));
diff --git a/src/bun.js/test/jest.zig b/src/bun.js/test/jest.zig
index 727466835..55600ded8 100644
--- a/src/bun.js/test/jest.zig
+++ b/src/bun.js/test/jest.zig
@@ -333,7 +333,7 @@ pub const Jest = struct {
     pub fn Bun__Jest__createTestModuleObject(globalObject: *JSC.JSGlobalObject) callconv(.C) JSC.JSValue {
         JSC.markBinding(@src());
 
-        const module = JSC.JSValue.createEmptyObject(globalObject, 12);
+        const module = JSC.JSValue.createEmptyObject(globalObject, 13);
 
         const test_fn = JSC.NewFunction(globalObject, ZigString.static("test"), 2, TestScope.call, false);
         module.put(
@@ -431,17 +431,40 @@ pub const Jest = struct {
             Expect.getConstructor(globalObject),
         );
 
+        const setSystemTime = JSC.NewFunction(globalObject, ZigString.static("setSystemTime"), 0, JSMock__jsSetSystemTime, false);
+        module.put(
+            globalObject,
+            ZigString.static("setSystemTime"),
+            setSystemTime,
+        );
+        const useFakeTimers = JSC.NewFunction(globalObject, ZigString.static("useFakeTimers"), 0, JSMock__jsUseFakeTimers, false);
+        const useRealTimers = JSC.NewFunction(globalObject, ZigString.static("useRealTimers"), 0, JSMock__jsUseRealTimers, false);
+
         const mockFn = JSC.NewFunction(globalObject, ZigString.static("fn"), 1, JSMock__jsMockFn, false);
         const spyOn = JSC.NewFunction(globalObject, ZigString.static("spyOn"), 2, JSMock__jsSpyOn, false);
         const restoreAllMocks = JSC.NewFunction(globalObject, ZigString.static("restoreAllMocks"), 2, JSMock__jsRestoreAllMocks, false);
         module.put(globalObject, ZigString.static("mock"), mockFn);
 
-        const jest = JSValue.createEmptyObject(globalObject, 3);
+        const jest = JSValue.createEmptyObject(globalObject, 7);
         jest.put(globalObject, ZigString.static("fn"), mockFn);
         jest.put(globalObject, ZigString.static("spyOn"), spyOn);
         jest.put(globalObject, ZigString.static("restoreAllMocks"), restoreAllMocks);
+        jest.put(
+            globalObject,
+            ZigString.static("setSystemTime"),
+            setSystemTime,
+        );
+        jest.put(
+            globalObject,
+            ZigString.static("useFakeTimers"),
+            useFakeTimers,
+        );
+        jest.put(
+            globalObject,
+            ZigString.static("useRealTimers"),
+            useRealTimers,
+        );
         jest.put(globalObject, ZigString.static("now"), JSC.NewFunction(globalObject, ZigString.static("now"), 0, JSMock__jsNow, false));
-        jest.put(globalObject, ZigString.static("setSystemTime"), JSC.NewFunction(globalObject, ZigString.static("setSystemTime"), 0, JSMock__jsSetSystemTime, false));
 
         module.put(globalObject, ZigString.static("jest"), jest);
         module.put(globalObject, ZigString.static("spyOn"), spyOn);
@@ -462,6 +485,8 @@ pub const Jest = struct {
     extern fn JSMock__jsSetSystemTime(*JSC.JSGlobalObject, *JSC.CallFrame) JSC.JSValue;
     extern fn JSMock__jsRestoreAllMocks(*JSC.JSGlobalObject, *JSC.CallFrame) JSC.JSValue;
     extern fn JSMock__jsSpyOn(*JSC.JSGlobalObject, *JSC.CallFrame) JSC.JSValue;
+    extern fn JSMock__jsUseFakeTimers(*JSC.JSGlobalObject, *JSC.CallFrame) JSC.JSValue;
+    extern fn JSMock__jsUseRealTimers(*JSC.JSGlobalObject, *JSC.CallFrame) JSC.JSValue;
 
     pub fn call(
         _: void,
diff --git a/test/js/bun/test/test-timers.test.ts b/test/js/bun/test/test-timers.test.ts
index 64da0abda..963467dee 100644
--- a/test/js/bun/test/test-timers.test.ts
+++ b/test/js/bun/test/test-timers.test.ts
@@ -1,11 +1,28 @@
 test("we can go back in time", () => {
-  const dateNow = Date.now;
-  //   jest.useFakeTimers();
-  jest.setSystemTime(new Date("2020-01-01T00:00:00.000Z"));
-  expect(new Date().toISOString()).toBe("2020-01-01T00:00:00.000Z");
-  expect(Date.now()).toBe(1577836800000);
-  expect(dateNow).toBe(Date.now);
-
-  jest.setSystemTime();
-  expect(new Date().toISOString()).not.toBe("2020-01-01T00:00:00.000Z");
+  const DateBeforeMocked = Date;
+  const orig = new Date();
+  orig.setHours(0, 0, 0, 0);
+  jest.useFakeTimers();
+  jest.setSystemTime(new Date("1995-12-19T00:00:00.000Z"));
+
+  expect(new Date().toISOString()).toBe("1995-12-19T00:00:00.000Z");
+  expect(Date.now()).toBe(819331200000);
+
+  if (typeof Bun !== "undefined") {
+    // In bun, the Date object remains the same despite being mocked.
+    // This prevents a whole bunch of subtle bugs in tests.
+    expect(DateBeforeMocked).toBe(Date);
+    expect(DateBeforeMocked.now).toBe(Date.now);
+
+    // Jest doesn't property mock new Intl.DateTimeFormat().format()
+    expect(new Intl.DateTimeFormat().format()).toBe("12/19/1995");
+  } else {
+    expect(DateBeforeMocked).not.toBe(Date);
+    expect(DateBeforeMocked.now).not.toBe(Date.now);
+  }
+
+  jest.useRealTimers();
+  const now = new Date();
+  now.setHours(0, 0, 0, 0);
+  expect(now.toISOString()).toBe(orig.toISOString());
 });