The display of types
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The display of types
Effective TypeScript: The display of types
https://effectivetypescript.com/2022/02/25/gentips-4-display/
type Resolve<T> = T extends Function ? T : {[K in keyof T]: T[K]} seems to be able to preserve types like number or "42":
type n = Resolve<number> // number
type s42 = Resolve<"42"> // "42"How is that even possible? Ahah I mean {[K in keyof "42"]: "42"[K]} is definitely not the same as "42".
It is surprising, isn't it? One clue is that keyof gives the methods defined on the wrapper types for primitives:
type K = keyof number;
// ^? type K = "toString" | "toFixed" | "toExponential" | ...But still, I think this must be special-cased in the TypeScript compiler. Especially in the case of a literal type, the result is much more specific than you'd expect. Resolve is magical indeed!
I think I found something: microsoft/TypeScript#12447
{[K in keyof T]: T[K]}is what we now call a homomorphic (previously isomorphic) mapped type, and ahejlsberg said that "when a primitive type is substituted for T in an isomorphic mapped type, we simply produce that primitive type".