Cannot use class before definition

[letmaik]

TypeScript Version: 2.0

Code

new A()
class A {}

Expected behavior:
The code should run without error.
EDIT: The code is invalid, see below. Expected behavior is a compile-time error.

Actual behavior:
The first line throws a "TypeError: A is not a constructor".
This is because class A {} is transpiled to var A = ... and kept in the same code order. If A was transpiled as function A () {...} then it would work as function statements are hoisted but function expressions are not.

Babel and traceur do the same thing, so maybe this is just a hard issue and not worth solving as eventually there will be native class support anyway in engines.

The problem becomes more obscure when doing things like:

class B extends A {}
class A {}

Same issue but the error changes to a weird TypeError: b is undefined.

I think either this should be solved or alternatively a compile-time error thrown if the target is <ES2015.

EDIT: As detailed in #5207 class declarations are actually not hoistable (see ES2015 spec), so the only problem here is that there is no compile-time error and the code above is actually invalid ES2015 code.

[kitsonk]

Duplicate of #5207

[letmaik]

Thanks, I didn't see that. I edited the issue text as I was clearly wrong in assuming that class declarations are hoistable where in fact they are not.

[kitsonk]

Yeah, it was a hard one to find, but I remembered it being brought up before (and I thought I had remembered about them not being hoistable too)