Prohibit faking of faked timers

src/fake-timers-src.js

@@ -425,6 +425,7 @@ function withGlobal(_global) {
         target.parse = source.parse;
         target.UTC = source.UTC;
         target.prototype.toUTCString = source.prototype.toUTCString;
+        target.isFake = true;
 
         return target;
     }
@@ -1635,6 +1636,14 @@ function withGlobal(_global) {
             );
         }
 
+        if (_global.Date.isFake === true) {
+            // Timers are already faked; this is a problem.
+            // Make the user reset timers before continuing.
+            throw new TypeError(
+                "Can't install fake timers twice on the same global object."
+            );
+        }
+
         // eslint-disable-next-line no-param-reassign
         config = typeof config !== "undefined" ? config : {};
         config.shouldAdvanceTime = config.shouldAdvanceTime || false;

test/fake-timers-test.js

@@ -53,6 +53,81 @@ const utilPromisifyAvailable = promisePresent && utilPromisify;
 const timeoutResult = global.setTimeout(NOOP, 0);
 const addTimerReturnsObject = typeof timeoutResult === "object";
 
+describe("issue #2449: permanent loss of native functions", function () {
+    it("should not fake faked timers", function () {
+        const currentTime = new Date().getTime();
+        const date1 = new Date("2015-09-25");
+        const date2 = new Date("2015-09-26");
+        let clock = FakeTimers.install({ now: date1 });
+        assert.same(clock.now, date1.getTime());
+        assert.same(new Date().getTime(), 1443139200000);
+        assert.exception(function () {
+            FakeTimers.install({ now: date2 });
+        });
+        clock.uninstall();
+        clock = FakeTimers.install({ now: date2 });
+        assert.same(clock.now, date2.getTime());
+        clock.uninstall();
+        assert.greater(new Date().getTime(), currentTime, true);
+    });
+
+    it("should not fake faked timers on a custom target", function () {
+        const setTimeoutFake = sinon.fake();
+        const context = {
+            Date: Date,
+            setTimeout: setTimeoutFake,
+            clearTimeout: sinon.fake(),
+        };
+        let clock = FakeTimers.withGlobal(context).install();
+        assert.exception(function () {
+            clock = FakeTimers.withGlobal(context).install();
+        });
+        clock.uninstall();
+
+        // After uninstaling we should be able to install without issue
+        clock = FakeTimers.withGlobal(context).install();
+        clock.uninstall();
+    });
+
+    it("should not allow a fake on a custom target if the global is faked and the context inherited from the global", function () {
+        const globalClock = FakeTimers.install();
+        assert.equals(new Date().getTime(), 0);
+        const setTimeoutFake = sinon.fake();
+        const context = {
+            Date: Date,
+            setTimeout: setTimeoutFake,
+            clearTimeout: sinon.fake(),
+        };
+        assert.equals(new context.Date().getTime(), 0);
+        assert.exception(function () {
+            FakeTimers.withGlobal(context).install();
+        });
+
+        globalClock.uninstall();
+        refute.equals(new Date().getTime(), 0);
+    });
+
+    it("should allow a fake on the global if a fake on a customer target is already defined", function () {
+        const setTimeoutFake = sinon.fake();
+        const context = {
+            Date: Date,
+            setTimeout: setTimeoutFake,
+            clearTimeout: sinon.fake(),
+        };
+        const clock = FakeTimers.withGlobal(context).install();
+        assert.equals(new context.Date().getTime(), 0);
+        refute.equals(new Date().getTime(), 0);
+        const globalClock = FakeTimers.install();
+        assert.equals(new Date().getTime(), 0);
+
+        globalClock.uninstall();
+        refute.equals(new Date().getTime(), 0);
+        assert.equals(new context.Date().getTime(), 0);
+        clock.uninstall();
+        refute.equals(new Date().getTime(), 0);
+        refute.equals(new context.Date().getTime(), 0);
+    });
+});
 describe("issue #59", function () {
     it("should install and uninstall the clock on a custom target", function () {
         const setTimeoutFake = sinon.fake();
@@ -5139,11 +5214,13 @@ describe("loop limit stack trace", function () {
     });
 
     describe("setImmediate", function () {
-        beforeEach(function () {
+        before(function () {
             if (!setImmediatePresent) {
                 this.skip();
             }
+        });
 
+        beforeEach(function () {
             function recursiveCreateTimer() {
                 setImmediate(function recursiveCreateTimerTimeout() {
                     recursiveCreateTimer();