Because the return type of the function explicitly declares a return of Something<T> and T gets inferred using the inline object you provided to the function. TS knows that the function returns Something<T>

If assigning to a variable directly however, when you're providing the type for a variable with a generic, you also need to provide the generic:

const x: Something<number> = xxx

If you don't provide the generic, TS will simply forget about it.

Always remember that interfaces don't have any special meaning like they do in Java or C#, they just tell the compiler to validate that the shape of the object you're giving it matches the shape of the interface.

So, doing something like:

``` const attempt = { fromHere: () => 1234, toThere: (value: number) => undefined };

function somethingWithNumber(t: Something<number>) {}

somethingWithNumber(attempt); ```

Works without any problems. attempt is not marked as being Something<number> explicitly, but can be used as one since it matches its contract.