Promise.then() incorrectly returns the same Promise instance

[kpreisser]

Run the following code in Jurassic:

Promise.reject(-1).then(undefined, function () { return 1; }).then(function (e) {
    console.log('resolved: ' + e);
}, function (e) {
    console.log('rejected: ' + e);
});

Actual output:

rejected: -1

Expected output: (can be seen when running in browsers)

resolved: 1

According to MDN, Promise.then returns a new Promise instance whose state will depend on the handler function.

In this case, the onRejected handler added to the first Promise returns value 1, so the Promise returned by then should be a new instance that will be resolved with the value 1.

However, it seems then as implemented in Jurassic (by #156) incorrectly returns the current Promise instance instead of creating a new one: https://github.com/paulbartrum/jurassic/blob/0438d0f98e52731e4a1e62f20d66144d59824925/Jurassic/Library/Promise/PromiseInstance.cs#L333-L355

[kpreisser]

//cc @jcdickinson

[paulbartrum]

I started looking into this, but I found a bunch more issues. I'll keep working on it.

[jcdickinson]

@paulbartrum let me know if you want me to pick this up.

[paulbartrum]

I've checked in a fix for this issue, let me know if you notice anything wrong with it.