ClockDate class overwrite prototype of instance

[Oustinger]

ClockDate class overwrite prototype of instance.
In my real project it happens when I use datepicker library (@eonasdan/tempus-dominus).

• FakeTimers version : 9.1.2
• Environment : Opera 89.0.4447.64, Windows 10
• Example URL : repository
• Other libraries you are using: -

What did you expect to happen?
I expected to receive instance of DateTime with it's methods

What actually happens
Prototype of child instance is not equals to DateTime class, but equals to fake date class

How to reproduce
I'd created a repository with reproducing this problem

i. You can clone and run project
ii. Or you can create project with @sinonjs/fake-timers and run the code below by yourself

var FakeTimers = require("@sinonjs/fake-timers");

FakeTimers.install();

class DateTime extends Date {
    constructor() {
        super();

        this.bar = 'bar';
    }

    foo() {
        console.log('Lorem inpsum');
    }
}

var dateTime = new DateTime();

dateTime.foo();

[benjamingr]

This is probably a bug because when you new something in JS and return something different from the constructor - it will return that thing.

class Foo {
  constructor() {
     return { x: 5 };
  }
  bar() {} 
}
const f = new Foo();
f.bar(); 

We should probably fix the code to not do that or call Reflect.construct(new.target) instead of assuming we have a NativeDate.

[fatso83]

when you new something in JS and return something different from the constructor

Where do we do that? I could not see this happening in the example code.

[benjamingr]

Where do we do that? I could not see this happening in the example code.

Our ClockDate function doesn't use this and instead has a return value even for the case it doesn't return a native date.

[fatso83]

@Oustinger The quickest path to seeing a patch commit for this is probably submit a small PR with a fix and regression test. I can make a release quick, but I do not have time atm to debug, fix and verify this.

[Oustinger]

@fatso83 I would do that. But I couldn't find a place to fix on my own.

[fatso83]

No one would expect you to 🙂 I certainly did not. Do you think you have the hints you need?

[Oustinger]

Where is a place in code, where new date's prototype becomes equal to ClockDate?
My debugger can't go deeper this line. What is going on after that?

[fatso83]

You cannot get deeper than that function, but the setup of the prototype happens before that

https://github.com/sinonjs/fake-timers/blob/main/src/fake-timers-src.js#L446-L489 covers all the return bits where we create NativeDate instances and line 491 is where return our own mirrored constructor. That function call sets up the prototype.

We should probably fix the code to not do that

Not totally sure how to fix the code myself TBH. ~I am guessing we are invoking NativeDate to create Date objects that are identical in behavior to the NativeDate, and if we were not to invoke NativeDate directly I am not sure what to do. ~

This fixed the issue when I tested:

                        //return new NativeDate(ClockDate.clock.now);
                        NativeDate.call(this,ClockDate.clock.now);
                        return;

Not sure if this is a general fix, but try and see if the test suite works when doing it for all paths :)

[stale]

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