Comments (2)
It doesn't look like reduce
itself is generic. The point of inference is Object.values
where it infers T
as number | null
from Record<string, number | null>
, which is correct.
Correction: I missed the third overload, which is generic. It looks like that overload is not being selected here. Probably the first overload (non-generic) is being selected since your arguments are all valid subtypes of the parameter types.
The error goes away if you explicitly call the generic overload.
const sum1 = Object.values(A).reduce<number>((sum) => sum + 1, 0);
from typescript.
Probably the first overload (non-generic) is being selected since your arguments are all valid subtypes of the parameter types.
That's what's going on. I think the ordering priority of the overloads is justified. If you call reduce
with an initialValue
that is compatible with the element type of the array, isn't it more likely that you want that element type and not a subtype?
One can easily devise examples of reasonable-looking operations that fail if the overloads are switched: Playground
from typescript.
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from typescript.