Use intersections to filter unions

filterUsingIntersections.ts

type Decision = "yes" | "no" | 1 | 0 | true | false;
type JustStrings = Decision & string;
///              = ("yes" | "no" | 1 | 0 | true | false) & string
///              = | ("yes" & string) | ("no" & string)
///                | (1 & string) | (0 & string)
///                | (true & string)| (false & string))
///              = "yes" | "no" | never | never | never | never
///              = "yes" | "no"

//
// when mapping types, if I want to extract the string keys I generally prefer & string to Extract
type GetMethods<T extends object> = {
  [P in keyof T & string as `get${Capitalize<P>}`]: () => T[P];
};

type R = GetMethods<{ foo: string; 0: number }>;
//     = { getFoo: () => string; }