Confusing Typescript Generic Typing Error

Question

I'm really confused here. This is also an extension of this if you need any extra detail.

This is my code:

interface Collections extends Twitter.Collections, Coin.Collections {}
type CollectionName = keyof Collections
type CollectionType <T extends CollectionName> = Collections[T]

const _collections: {
    [K in CollectionName]?: Collection<CollectionType<K>>
} = {}

export async function getCollection<T extends CollectionName> (name: T): Promise<Collection<CollectionType<T>>> {
    if (name in _collections && typeof _collections[name] !== 'undefined') {
        return _collections[name]
    }

    ...
}

It's on this last if line that the following ts error displays:

Type '{ "twitter:tweets"?: Collection<Tweet> | undefined; "twitter:users"?: Collection<User> | undefined; "twitter:metadata-cashtag"?: Collection<CashtagMetadataDb> | undefined; "coins:all"?: Collection<...> | undefined; }[T]' is not assignable to type 'Collection<Collections[T]>'.
    Type 'Collection<Tweet> | Collection<User> | Collection<CashtagMetadataDb> | Collection<Coin> | undefined' is not assignable to type 'Collection<Collections[T]>'.
        Type 'undefined' is not assignable to type 'Collection<Collections[T]>'.

As you can see, I've tried my best to type check here but I'm unable to get this to work.

Thanks for your help :)

The code provided is not enough to reproduce the error. But I think it's related to [K in CollectionName]? being optional. It could be fixed using definite assertions return _collections[name]! (note the exclamation mark), but again I'm not sure since the code provided is not enough — lleon
– lleon, Commented Aug 31, 2018 at 9:41
And I don't know if you're returning the correct type after the if clause. So this could be another error — lleon
– lleon, Commented Aug 31, 2018 at 9:47
When I remove the ? in [K in CollectionName]? that error goes away but I can't fill the object on declaration. I'll have to use the bang, !, which I never knew about before. Thanks for your help :) — James Middleton
– James Middleton, Commented Aug 31, 2018 at 15:38

Matt McCutchen · Accepted Answer · 2018-08-31 22:04:34Z

1

Automatic narrowing doesn't work on element access expressions like _collections[name]. If you want to take advantage of narrowing, you'll have to save the value in a local variable before testing it. And due to limitations on how TypeScript reasons about lookup types, you'll need a type annotation on that local variable. This should work:

export async function getCollection<T extends CollectionName>(name: T): Promise<Collection<CollectionType<T>>> {
    let coll: Collection<CollectionType<T>> | undefined = _collections[name];
    if (coll !== undefined) {
        return coll
    }
    // ...
}

answered Aug 31, 2018 at 22:04

Matt McCutchen

31.4k3 gold badges76 silver badges82 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Matt McCutchen Over a year ago

Can you please collect the complete code to reproduce the error in the question so I can debug?

Collectives™ on Stack Overflow

Confusing Typescript Generic Typing Error

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related