TypeScriptのExtract型で型の共通部分を抽出してみよう

type Common = Extract<"a" | "b" | "c", "b" | "c" | "d">;
// Commonは"b" | "c"になります

type UnionA = "x" | "y" | "z";
type UnionB = "y" | "z" | "w";
type Shared = Extract<UnionA, UnionB>;
// Sharedは"y" | "z"になります

type Status = "success" | "error" | "pending";
type AllowedStatus = "success" | "error";
type ValidStatus = Extract<Status, AllowedStatus>;
// ValidStatusは"success" | "error"になります

type Letters = "a" | "b" | "c" | "d";
type Vowels = "a" | "e" | "i" | "o" | "u";
type CommonLetters = Extract<Letters, Vowels>;
// CommonLettersは"a"になります

type IsString<T> = T extends string ? T : never;
type ExtractedString = Extract<"a" | 1 | "b" | 2, string>;
// ExtractedStringは"a" | "b"になります

function extractValues<T, U>(values: T[], filter: U): Extract<T, U>[] {
  return values.filter(value => value === filter) as Extract<T, U>[];
}

const items = ["a", "b", "c"] as const;
const result = extractValues(items, "a");
// resultは["a"]になります

type Role = "admin" | "user" | "guest";
type PrivilegedRole = "admin" | "user";
type AccessibleRoles = Extract<Role, PrivilegedRole>;
// AccessibleRolesは"admin" | "user"になります

type Numbers = 1 | 2 | 3 | 4 | 5;
type EvenNumbers = Extract<Numbers, 2 | 4>;
// EvenNumbersは2 | 4になります

const example = { name: "Alice", age: 30 };
type ExampleType = typeof example;
type StringProperties = Extract<keyof ExampleType, "name" | "address">;
// StringPropertiesは"name"になります

type ApiResponse = { id: number; name: string; email: string };
type PublicResponse = Extract<keyof ApiResponse, "id" | "name">;
// PublicResponseは"id" | "name"になります

type Props = { id: number; name: string; onClick: () => void };
type EventHandlers = Extract<keyof Props, "onClick">;
// EventHandlersは"onClick"になります