[mattigames]

You have to watch out for first and last one in JavaScript but not on TypeScript as it isn't possible to make that mistake because you have to type it as a promise or in the last one as void.

You can even avoid the problem in the second one by using NonNullable TypeScript types, but I admit that's not common so its still likely to arise.

[achou]

The first example can happen in TypeScript; foo has type

  (() => Promise<void>) | undefined

admittedly it may not be all that common to have a function-valued variable that may be undefined, but it happened in the code base I was working with.

In the last example, you're right that TypeScript will catch this at compile time. My point was to show how this compile time error can happen from refactoring to use optional chaining, and one easy solution in this case.