CodeParktype Mutable<T> = { -readonly [K in keyof T]-?: T[K] };
type U = Mutable<{ readonly id: string; nickname?: string }>;
{ id: string; nickname: string }The answer is { id: string; nickname: string }. -readonly removes readonly, and -? removes the optional ?. Removing ? also drops the undefined that the ? added. So nickname is plain string, the same behaviour as Required.{ id: string; nickname: string | undefined } is the tempting wrong answer, because it keeps that undefined.
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