Fake timers override mocks on performance object

• FakeTimers version : I am using jest: 26.6.3 which uses FakeTimers
• Environment : Windows 10 10.0.19042
• Other libraries you are using: jest 26.6.3

I am actually redirected here from https://github.com/facebook/jest/issues/11330 I am using jest with jsdom

What did you expect to happen? I would expect that properties that I added to be preserved after fake timers install

What actually happens performance object is overriden and properties I added before install are gone

How to reproduce I will explain how I did it in jest which they say they just only install FakeTimers and don’t do any logic Please check this https://github.com/facebook/jest/issues/11330

I have global mocks file where before testing I do this

Object.defineProperties(performance, {
    timing: {
        value: {
            connectStart: now + 1,
            connectEnd: now + 1,
            domComplete: now + 100,
            domContentLoadedEventEnd: now + 50,
            domContentLoadedEventStart: now + 40,
            domInteractive: now + 39,
            domLoading: now + 10,
            domainLookupStart: now + 1,
            domainLookupEnd: now + 1,
            fetchStart: now + 1,
            loadEventEnd: now + 1000,
            loadEventStart: now + 1000,
            navigationStart: now,
            redirectEnd: 0,
            redirectStart: 0,
            requestStart: now + 1,
            responseStart: now + 2,
            responseEnd: now + 30,
            secureConnectionStart: 0,
            unloadEventEnd: 0,
            unloadEventStart: 0
        },
        writable: true
    },
    navigation: {
        value: {
            type: 0
        },
        writable: true
    },
    getEntries: {
        value: (): PerformanceEntry[] => {
            return [];
        },
        writable: true
    },
    getEntriesByType: {
        value: (type: string): (PerformanceNavigationTiming | PerformanceResourceTiming)[] => {
            return performanceObj.filter(perf => {
                return perf.entryType === type;
            });
        },
        writable: true
    },
    getEntriesByName: {
        value: (): PerformanceEntry[] => {
            return [];
        },
        writable: true
    },
    setResourceTimingBufferSize: {
        value: jest.fn(),
        writable: true
    },
    clearResourceTimings: {
        value: jest.fn(),
        writable: true
    }
});

My app relies on these values. After I install FakeTimers the performance.timing is gone With jest legacy timers (which are not sinon/FakeTimers) this is not happening

From what I get is that FakeTimers mock performance.now() etc… but is it possible to preserve values that were added to performance object before clock install?

[Smrtnyk]

I do understand that you need to overwrite performance.now() due to timers controll in sync manner, but can you explain why clock.performance[‘getEntries’] = NOOP_ARRAY; clock.performance[‘getEntriesByName’] = NOOP_ARRAY; clock.performance[‘getEntriesByType’] = NOOP_ARRAY; need to be overriden? In my case I mock these in my global setup that happens before tests run (they do not give anything in jsdom) so I can have some mock resources to work in my tests. In your proposal after I install the clock it will override the spies and I will need to remock them again

[itayperry]

I’ll take a look into it - I might ask for some advice 😃 Hope I’ll be able to help! 🙂