example.ts

type Example<T> = T extends Record<string, any> ? keyof T : T;
// type T1 = number | keyof string[]
type T1 = Example<string[]>;
// type T2 = "a" | "b"
type T2 = Example<{ a: number; b: string }>;
// type T3 = boolean
type T3 = Example<boolean>;