typescript type operate

extract-returntype-from-async-function.ts

const x = async (y) => {
 return 1
}

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

export type T = Unpacked<ReturnType<typeof x>> // number

get-type-from-tuple-types.ts

// Get the last item type from a tuple of types

type Stuff = [number, boolean, string]
type OtherStuff = [string, number, object, boolean]

type LastStuff = Stuff[2] // => string
type LastOtherStuff = OtherStuff[3] // => boolean

type GetLength<original extends any[]> = original extends { length: infer L } ? L : never

type LengthStuff = GetLength<Stuff> // => 3
  
// Borrowed from SimplyTyped:
type Prev<T extends number> = [-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62][T];

// Actual, legit sorcery
// Borrowed from pelotom/hkts:
type GetLength<original extends any[]> = original extends { length: infer L } ? L : nevertype GetLast<original extends any[]> = original[Prev<GetLength<original>>]

// Here are our test-subject tuples:
type Stuff = [number, boolean, string]
type OtherStuff = [string, number, object, boolean]

// How long is the `Stuff` tuple?
type LengthStuff = GetLength<Stuff> // => 3

// What is the last element of each tuple?
type LastStuff = GetLast<Stuff> // => string
type LastOtherStuff = GetLast<OtherStuff> // => boolean

get-union-type-from-interface.ts

type a = { name: string; sex: boolean };

type Omit<T, K extends keyof T> = Pick<T, Exclude<keyof T, K>>

type KnownKeys<T> = {
    [K in keyof T]: string extends K ? never : number extends K ? never : K
} extends {[_ in keyof T]: infer U} ? U : never;

type b = KnownKeys<a>;

util.ts

// 获取参数的返回值类型
type GetReturnType<original extends Function> = 
  original extends (...x: any[]) => infer returnType ? returnType : never

const someRandomStuff = <fn extends Function>(originalFn: fn) => {
  const result: GetReturnType<fn> = originalFn(12345);

  return result;
}

const innerFn = (item: number) => item.toString();

const output = someRandomStuff(innerFn); // works greate

// 高级用法：获取reducer的类型
const reducer = {
  homepage, // (state, action) => IHomepageState;
};

type GetReturnType<original extends Function> = 
  original extends (...x: any[]) => infer returnType ? returnType : never

export type IFnReturnType<T> = {
  [P in keyof T]: T[P] extends Function ? GetReturnType<T[P]> : T[P];
};

let b: IFnReturnType<reducer>; // { homepage: IHomepageState }

函数式编程，使用替代类型来获得接下来函数的返回值类型