Why is IValueDidChange using unknown-property values?

[misantronic]

The current type-interface for IValueDidChange looks like this:

type IValueDidChange<T = any> = {
    type: "update";
    observableKind: "value";
    object: IObservableValue<T>;
    debugObjectName: string;
    newValue: unknown;
    oldValue: unknown;
}

In an older version of mobx, this was typed so that newValue and oldValue would use T | undefined as their default type which in my opinion makes total sense. Why is it now unknown?

[emereum]

This change also impacts us in a large codebase. It seems like this was done out of concern that a value could escape the type system, but I don't see how that is possible given IObservableValue<T>'s very narrow API surface:

export interface IObservableValue<T> {
    get(): T;
    set(value: T): void;
    intercept_(handler: IInterceptor<IValueWillChange<T>>): Lambda;
    observe_(listener: (change: IValueDidChange<T>) => void, fireImmediately?: boolean): Lambda;
}

It would be helpful if the maintainers can advise what risk was mitigated by this change. This information will help steer our implementation of additional runtime type checking to resolve the new type errors.

[mweststrate]

The unknown looks really strange indeed, and I can't find any history back that this was intentionally changed, so feel to change it back. I'm wondering if this is strictness implemented wrong, and that the actual thing that should have happened is changing the generic default from T = any to T = unknown. Feel free to submit a PR.

[emereum]

PR submitted #3609