Repository navigation
Typing the Index Signature of a Module Namespace Object #10998
Description
Hi. I have a series of related classes packaged up and exported from a single module. Something like:
// states.ts
export {default as Alabama} from "./alabama";
export {default as Alaska} from "./alaska";
//...Now, elsewhere in the code, I want to iterate over all the related classes:
import * as States from "./states.ts"
for(let stateExportName in States) {
let thisState = States[stateExportName];
// ... do something.
}Hold aside that iteration over a commonjs module's export names would use a for-in, whereas iterating over an es6 module namespace object's exports would use a for-of (since es6 does define @Iterator (@iterator) on module namespace objects)... in either case, this iteration should be possible.
I'd like to be able to provide a type for the index signature of the module namespace object, so that thisState is automatically inferred as a State. I asked on StackOverflow, and Basarat Ali Syed (@basarat) said that wasn't possible, so I'm opening an issue here for it as a potential feature request. It does seem like an edge case, so I'm not sure how worth solving it is, but it is something I bumped into.
The other thing is that, if I use the code as written above, I get an "Index signature of object type implicitly has an 'any' type" error. That makes sense, since I haven't defined an index signature for the module anywhere. However, I get that error even if I explicitly type the thisState variable as any like so:
for(let stateExportName in States) {
let thisState: any = States[stateExportName];
// ... do something.
}I can't tell if that's a bug or expected behavior. If it is expected behavior, though, then I have to do some trickery to silence the compiler error...which is really annoying (aesthetically—but also because having any compiler errors seems to prevent tslint from running with type-checking on).
Activity
mhegazy commentedon Sep 19, 2016
possibly something like the prosal in #420 could cover this.
Another possibility is using something like
keysofoperator (#10425) to:RyanCavanaugh commentedon Sep 19, 2016
To remove the implicit any error, what you want to write is
ethanresnick commentedon Sep 20, 2016
Ooh, that seems cool. And maybe
keysof Statescould be automatically inferred as the type in the for-in loop? (I can see why inferringkeysof argmight not work in the general case for for-in loops, because of properties up the prototype chain...but it does seem like it should work for module namespace objects, since I'm pretty sure their keys are only own keys, and those can be known statically.)Thanks Ryan Cavanaugh (@RyanCavanaugh)! That's the one permutation I didn't try :P
kitsonk commentedon Jul 26, 2017
Ran across a similiar problem where some sort of inferrance of the index signature would be great. Trying to "rollup" some modules into a larger map is currently not possible.
Here is an example of the problem we are facing. Given a.ts:
And then in b.ts:
Produces
Index signature is missing in type 'typeof "a"'.and I can't figure out an easy way around it, even though TypeScript could infer an index type for the import. The only sort of way that would work would be to cast toanybefore casting to an index type, but that is type abuse, and I hate being cruel to types.kitsonk commentedon Aug 18, 2017
For sanity purposes, as it seems there are at least 3 open issues on this, Ryan Cavanaugh (@RyanCavanaugh) indicates that an implicit index type can be inferred.
RyanCavanaugh commentedon Jun 23, 2021
This is possible today if the indexing type is
keyofthe containing object. Inferring an index signature would be problematic due to it not being legal to write to additional new keys, so I think the current behavior is what we want.