Dates created before useFakeTimers aren't seen as instanceof Date anymore

[erossignon]

• FakeTimers version : @sinonjs/fake-timers@13.0.1
• Environment : nodejs
• Example URL :
• Other libraries you are using: node-opcua, sinon@19.0.0

What did you expect to happen?

Date object that have been created before fakeTimer takes actions should still be instanceof Date, as it used to be in previous versions

What actually happens

How to reproduce

npm install sinon@19

 import sinon from "sinon";


 // defined before useFakeTimers
 const minDate = new Date(Date.UTC(1601, 0, 1, 0, 0, 0));
 console.log("minDate instanceof Date (before)", minDate instanceof Date);
 const clock = sinon.useFakeTimers();
 const d = new Date();
 console.log("instanceof Date", d instanceof Date);

 console.log("minDate instanceof Date (after) : should be true but is : ", minDate instanceof Date);

 clock.restore();

// without using sinon
import FakeTimers from "@sinonjs/fake-timers";

// emulating what sinonjs is doing:
function createClock(config, globalCtx) {
    let FakeTimersCtx = FakeTimers;
    if (globalCtx !== null && typeof globalCtx === "object") {
        FakeTimersCtx = FakeTimers.withGlobal(globalCtx);
    }
    const clock = FakeTimersCtx.install(config);
    clock.restore = clock.uninstall;
    return clock;
}

 // defined before useFakeTimers
 const minDate = new Date(Date.UTC(1601, 0, 1, 0, 0, 0));
 console.log("minDate instanceof Date (before)", minDate instanceof Date);
 
var clock = createClock();
 const d = new Date();
 console.log("instanceof Date", d instanceof Date);

 console.log("minDate instanceof Date (after) : should be true but is : ", minDate instanceof Date);

 clock.restore();

[fatso83]

While this is a difference, I am not sure this is a bug or something we should fix up. The change was published as a breaking change, and usually you would install the fake timers before running the tests, run the tests, and then restore things. This would be an issue if you create data used in the tests before installing fake timers, and I am not sure if that's something that is more important to support than supporting instanceof checks on subclasses, which seems like a more real use case to support.

Thoughts?

[erossignon]

also, in some cases, we may have well-defined dates that are global const. (therefore, created before FakeTimer takes action)

We may also need to test that a variant value is of type Date at some point and one of those well-defined date could surface.

̀if (variant instanceof Date) { doSomethingWithDate(variant as Date) }

[fatso83]

I did not think it was possible to do anything about this, but Ben and the symbols of ES2015 came to the rescue. You want to test that my proposed fix works for you? #505

I still regard using instanceof as a bit of an anti-pattern in a language as suitable for duck-typing as javascript, but this should satisfy all inclinations 😄

[fatso83]

Try the latest 19.0.2 version of Sinon. It should have the fix.

[erossignon]

Awesome solution , thanks,
I offer you PR #506 with the missing unit-test.

[fatso83]

The test was beautiful, but was already covered: I only linked to the specific commit above that had the fix and the test was in a previous commit. You can see it here:

ae0a266#diff-5a5796b4730f7629082606dc9407d4b8a084ab5ec38803e6141743b4bbb9f7efR3373