type instability of the newValue change argument

[jasongrout]

The type instability of the newValue and oldValue arguments to an event is frustrating. It means that pretty much all event handlers need to cast newValue and oldValue (like https://github.com/jasongrout/jupyter-js-notebook/blob/4cf0e81d019f13bb8992c68b57cc25f4fb5f27bf/src/NotebookWidget.ts#L103)

[sccolbert]

Do you have a suggestion for something better?

[sccolbert]

We could make newValue and oldValue just T[] instead of T | T[], which has precedence: https://msdn.microsoft.com/en-us/library/system.collections.specialized.notifycollectionchangedeventargs(v=vs.110).aspx

But I imagine that would just cause a different set of complaints.

[jasongrout]

Actually, I would prefer T[] just for consistency. I can imagine people not using types might complain that it is a bit harder, but newValue[0] is not too much harder than newValue.

[sccolbert]

It's also two extra memory allocations on every change, when they aren't strictly needed (and why I originally went this route)

[jasongrout]

On the other hand, if Typescript was smart enough to have type literals and the switch statement as a type guard, the problem would go away from the typescript perspective. That may be coming soon, at least for string literal typing.

[jasongrout]

Yeah, I was also wondering about efficiency.

[sccolbert]

newValue as T is only 2 characters longer than newValue[0]

[sccolbert]

and the same length as <T>newValue

but I guess if you are spelling out T it would be savings

[jasongrout]

See microsoft/TypeScript#1003 - const enum literals may solve the issue, in that typescript can be smarter about inferring types.