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Description
TypeScript Version: 3.2.0-dev.20181019
Search Terms: mapped tuples reify
Code
type Foo = ['a', 'b'];
interface Bar
{
a: string;
b: number;
}
type Baz = { [K in keyof Foo]: Bar[Foo[K]]; }; // Expected Baz to be [string, number]
Expected behavior: Baz should be [string, number]
Actual behavior: Type '["a", "b"][K]' cannot be used to index type 'Bar'.
Related Issues: #25947
Given the Mapped tuple types feature (#25947). I'd expect the code above to work cleanly.
However, I still need to do:
type WorkingBaz = { [K in Exclude<keyof Foo, keyof any[]>]: Foo[K] extends keyof Bar ? Bar[Foo[K]] : never; } & { length: Foo['length']; } & any[];
To have an equivalent type. As far as I understand, the "K" in a mapped type on a tuple should iterate only on numeric keys of this tuple. Therefore, Foo[K] should always be a valid key for Bar...