Typescript can't infer a generic type parameter from function arguments

Consider the following code:

type Collection<T = any> = T[]

type CollectionGetter = () => Collection

function collectionProcessor(getter: CollectionGetter) {
  const res = getter();
  // some processing...
  // return collection of the same type
  return res;
}

// ---

interface Item {
  id: number;
}

const myGetter = () => {
  return [
    {
      id: 1
    },
    {
      id: 2
    },
    {
      id: 3
    }
  ] as Collection<Item>
}

const result = collectionProcessor(myGetter);
// typeof result = Collection<any>
// expected: Collection<Item>
console.log(result);

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Typescript can’t infer the Collection<T> type parameter from the argument passed to collectionProcessor.

What is the right way to type this example?

I know I can type the processor like this function collectionProcessor<R>(getter: CollectionGetter): Collection<R> and pass the type explicitly collectionProcessor<Item>(myGetter), but that would not be very convenient as the argument is passed down from higher levels of abstraction.

>Solution :

You’re missing some generics to complete the chain.

type CollectionGetter<T> = () => Collection<T>

By omitting <T> here, you were infering only any.

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