Model's type parameter must be defined

[sandersn]

In Typescript 4.8 and mongodb since at least 4.8.1, mongodb.AnyBulkWriteOperation requires its type argument to be defined. It forbids null and undefined. Previous versions of Typescript didn't check this properly for unconstrained type parameters (lone type parameters like T). Typescript 4.8, which is in beta and will release in a week or two, does.

This PR adds a constraint to Model that requires its type parameter to be defined. It can no longer include null or undefined. In particular, that means Model<unknown> is no longer allowed; you have to write Model<{}> instead, which is the same type excluding null and undefined [1]. You can see this change in test/types/document.test.ts:179:

    expectType<Model<unknown>>(this.$model('Item1'));
// becomes
    expectType<Model<{}>>(this.$model('Item1'));

This change ripples out to other types as well, most notably the type parameter for Schema, which now also disallows null and undefined.

Alternative Fix

This is a pretty comprehensive PR. It should be possible to isolate the change to Model.bulkWrite by adding a conditional type:

mongodb.AnyBulkWriteOperation<T>
// would become
mongodb.AnyBulkWriteOperation<T extends {} ? T : any>

But this is unsafe since you can pass in writes that don't match Model's T, and because presumably AnyBulkWriteOperation really doesn't expect null or undefined and might crash at runtime.

I wouldn't recommend this smaller alternative except temporarily.

[1] unknown means "any value in Javascript". {} means "any value except null or undefined". object means "any non-primitive" (and does not allow null)

Recommendation

I'd recommend testing with typescript@beta during the Typescript beta period or, even better, typescript@next.

Fixes #12213

[sandersn]

I'm stuck trying to figure out how the type tests break, so I created #12221 for the small temporary fix, which will unblock people using Typescript 4.8 with mongoose.

[vkarpov15]

@sandersn sorry for the late reply. Why is this change necessary? It looks like Mongoose's types tests work fine with typescript@4.8.1-rc. Can you come up with a script that fails to compile on 4.8.1-rc1 but should succeed?

[sandersn]

I believe that the repro steps in #12213 still fail if you revert #12221 .

The problem is that #12221 makes typing less safe when the type parameter includes undefined. I believe it's not supposed to, but it would be better if all of the types correctly forbid undefined as with this PR. Unfortunately, I never got it to pass tests and ran out of time to work on it. You can close it if you don't want to pass it on to somebody else.

[vkarpov15]

I'll close for now. This doesn't seem like a huge problem, but I'm open to opposing viewpoints on that.