Improve type robustness for TrackedAsyncData<T>
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Export a union type as the actual public API of `TrackedAsyncData<T>`, along with a `const`-bound export of the class (with a safe cast for the constructor). The union is implemented as a union of interfaces which extend the base class so that (a) they are guaranteed to be structurally compatible and (b) docstrings continue to "just work". The upshot of this change: All existing invocations continue to work exactly as they have up to this point, but the type now correctly narrows when checking any of the boolean properties exposed to indicate the state of the type. For example, given a `TrackedAsyncData<string>`, the `.value` and `.error` properties will have the type `null` when `.isPending` is `true`; `.value` will have the type `string` when `.isResolved` is `true` but `.error` will still be `null`, etc.
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Export a union type as the actual public API of
TrackedAsyncData<T>, along with aconst-bound export of the class (with a safe cast for the constructor). The union is implemented as a union of interfaces which extend the base class so that (a) they are guaranteed to be structurally compatible and (b) docstrings continue to "just work".The upshot of this change: All existing invocations continue to work exactly as they have up to this point, but the type now correctly narrows when checking any of the boolean properties exposed to indicate the state of the type. For example, given a
TrackedAsyncData<string>, the.valueand.errorproperties will have the typenullwhen.isPendingistrue;.valuewill have the typestringwhen.isResolvedistruebut.errorwill still benull, etc.Thanks to @dfreeman for suggesting a variant on this approach!