TypeScript中infer关键字的具体使用

type SomeType<T> = T extends infer U ? U : never;

type ReturnType<T> = T extends (...args: any[]) => infer R ? R : never;

function foo(): number {
  return 42;
}

type FooReturn = ReturnType<typeof foo>; // number

type ElementType<T> = T extends (infer U)[] ? U : never;

type StrArray = string[];
type StrElement = ElementType<StrArray>; // string

type UnpackPromise<T> = T extends Promise<infer U> ? U : T;

type PromiseNumber = Promise<number>;
type NumberType = UnpackPromise<PromiseNumber>; // number

type Parameters<T> = T extends (...args: infer P) => any ? P : never;

function bar(a: string, b: number): void {}
type BarParams = Parameters<typeof bar>; // [string, number]

type InstanceType<T> = T extends new (...args: any[]) => infer R ? R : any;

class MyClass {}
type MyInstance = InstanceType<typeof MyClass>; // MyClass

type Reverse<T> = T extends [infer First, ...infer Rest] 
  ? [...Reverse<Rest>, First] 
  : T;

type Original = [1, 2, 3];
type Reversed = Reverse<Original>; // [3, 2, 1]