infer 关键字

// TypeScript 5.x

// 基本语法：在 extends 子句中使用 infer 声明类型变量
type GetReturnType<T> = T extends (...args: any[]) => infer R ? R : never

// 使用
function greet(): string {
  return 'hello'
}

type GreetReturn = GetReturnType<typeof greet>
// string

function add(a: number, b: number): number {
  return a + b
}

type AddReturn = GetReturnType<typeof add>
// number

// 非函数类型返回 never
type NotFunction = GetReturnType<string>
// never

// 提取返回值（内置 ReturnType 的实现）
type MyReturnType<T extends (...args: any) => any> = T extends (
  ...args: any
) => infer R
  ? R
  : any

type R1 = MyReturnType<() => string> // string
type R2 = MyReturnType<(x: number) => boolean> // boolean
type R3 = MyReturnType<typeof Math.random> // number

// 处理异步函数
type AsyncReturnType<T extends (...args: any) => Promise<any>> = T extends (
  ...args: any
) => Promise<infer R>
  ? R
  : never

async function fetchData() {
  return { id: 1, name: '张三' }
}

type Data = AsyncReturnType<typeof fetchData>
// { id: number; name: string }

// 提取参数类型（内置 Parameters 的实现）
type MyParameters<T extends (...args: any) => any> = T extends (
  ...args: infer P
) => any
  ? P
  : never

function example(name: string, age: number, active: boolean) {}

type Params = MyParameters<typeof example>
// [name: string, age: number, active: boolean]

// 提取第一个参数
type FirstParam<T extends (...args: any) => any> = T extends (
  first: infer F,
  ...rest: any
) => any
  ? F
  : never

type First = FirstParam<typeof example>
// string

// 提取剩余参数
type RestParams<T extends (...args: any) => any> = T extends (
  first: any,
  ...rest: infer R
) => any
  ? R
  : never

type Rest = RestParams<typeof example>
// [age: number, active: boolean]

// 提取最后一个参数
type LastParam<T extends any[]> = T extends [...any, infer L] ? L : never

type Last = LastParam<Parameters<typeof example>>
// boolean

// 提取 this 参数类型
type ThisParameterType<T> = T extends (this: infer U, ...args: any[]) => any
  ? U
  : unknown

function greet(this: { name: string }) {
  console.log(`Hello, ${this.name}`)
}

type GreetThis = ThisParameterType<typeof greet>
// { name: string }

// 移除 this 参数
type OmitThisParameter<T> = T extends (this: any, ...args: infer A) => infer R
  ? (...args: A) => R
  : T

type GreetWithoutThis = OmitThisParameter<typeof greet>
// () => void

// 提取数组元素类型
type ElementType<T> = T extends (infer E)[] ? E : never

type E1 = ElementType<string[]> // string
type E2 = ElementType<number[]> // number
type E3 = ElementType<(string | number)[]> // string | number

// 提取元组第一个元素
type Head<T extends any[]> = T extends [infer H, ...any[]] ? H : never

type H1 = Head<[string, number, boolean]> // string
type H2 = Head<[]> // never

// 提取元组最后一个元素
type Tail<T extends any[]> = T extends [...any[], infer L] ? L : never

type T1 = Tail<[string, number, boolean]> // boolean

// 提取除第一个外的元素
type Rest<T extends any[]> = T extends [any, ...infer R] ? R : never

type R1 = Rest<[1, 2, 3]> // [2, 3]
type R2 = Rest<[1]> // []

// 提取除最后一个外的元素
type Init<T extends any[]> = T extends [...infer I, any] ? I : never

type I1 = Init<[1, 2, 3]> // [1, 2]

// 提取 Promise 内部类型
type UnwrapPromise<T> = T extends Promise<infer U> ? U : T

type P1 = UnwrapPromise<Promise<string>> // string
type P2 = UnwrapPromise<Promise<number[]>> // number[]
type P3 = UnwrapPromise<string> // string（非 Promise 返回原类型）

// 深度解包（内置 Awaited 的实现）
type DeepAwaited<T> = T extends Promise<infer U> ? DeepAwaited<U> : T

type D1 = DeepAwaited<Promise<Promise<string>>> // string
type D2 = DeepAwaited<Promise<Promise<Promise<number>>>> // number

// 提取 PromiseLike
type UnwrapPromiseLike<T> = T extends PromiseLike<infer U> ? U : T

const thenable = {
  then: (resolve: (value: string) => void) => resolve('hello'),
}

type Thenable = UnwrapPromiseLike<typeof thenable>
// string

// 提取对象属性类型
type PropertyType<T, K extends keyof T> = T extends { [P in K]: infer V }
  ? V
  : never

interface User {
  id: number
  name: string
  email: string
}

type IdType = PropertyType<User, 'id'> // number

// 提取构造函数实例类型
type InstanceType<T extends new (...args: any) => any> = T extends new (
  ...args: any
) => infer R
  ? R
  : any

class User {
  constructor(public name: string) {}
}

type UserInstance = InstanceType<typeof User>
// User

// 提取构造函数参数
type ConstructorParameters<T extends new (...args: any) => any> =
  T extends new (...args: infer P) => any ? P : never

type UserParams = ConstructorParameters<typeof User>
// [name: string]

// 同时提取键和值
type KeyValue<T> = T extends { key: infer K; value: infer V } ? [K, V] : never

type KV = KeyValue<{ key: 'name'; value: string }>
// ['name', string]

// 提取函数的参数和返回值
type FunctionParts<T> = T extends (...args: infer P) => infer R
  ? { params: P; return: R }
  : never

type Parts = FunctionParts<(a: string, b: number) => boolean>
// { params: [a: string, b: number]; return: boolean }

// 提取元组的头尾
type HeadAndTail<T extends any[]> = T extends [infer H, ...infer T]
  ? { head: H; tail: T }
  : never

type HT = HeadAndTail<[1, 2, 3, 4]>
// { head: 1; tail: [2, 3, 4] }

// 根据类型选择推断方式
type Unpacked<T> = T extends (infer U)[]
  ? U
  : T extends (...args: any[]) => infer U
    ? U
    : T extends Promise<infer U>
      ? U
      : T

type T0 = Unpacked<string> // string
type T1 = Unpacked<string[]> // string
type T2 = Unpacked<() => string> // string
type T3 = Unpacked<Promise<string>> // string

// 嵌套解包
type DeepUnpacked<T> = T extends (infer U)[]
  ? DeepUnpacked<U>
  : T extends (...args: any[]) => infer U
    ? DeepUnpacked<U>
    : T extends Promise<infer U>
      ? DeepUnpacked<U>
      : T

type D0 = DeepUnpacked<Promise<string[]>> // string
type D1 = DeepUnpacked<() => Promise<number[]>> // number

// 将对象方法转换为异步版本
type AsyncMethods<T> = {
  [K in keyof T]: T[K] extends (...args: infer A) => infer R
    ? (...args: A) => Promise<R>
    : T[K]
}

interface UserService {
  getUser(id: number): User
  updateUser(user: User): void
  deleteUser(id: number): boolean
}

type AsyncUserService = AsyncMethods<UserService>
// {
//   getUser: (id: number) => Promise<User>
//   updateUser: (user: User) => Promise<void>
//   deleteUser: (id: number) => Promise<boolean>
// }

// 从路由路径提取参数
type ExtractRouteParams<T extends string> =
  T extends `${infer Start}:${infer Param}/${infer Rest}`
    ? { [K in Param | keyof ExtractRouteParams<Rest>]: string }
    : T extends `${infer Start}:${infer Param}`
      ? { [K in Param]: string }
      : {}

type Params1 = ExtractRouteParams<'/users/:id'>
// { id: string }

type Params2 = ExtractRouteParams<'/users/:userId/posts/:postId'>
// { userId: string; postId: string }

type Params3 = ExtractRouteParams<'/home'>
// {}

// 从组件类型提取事件处理器
type ExtractEventHandlers<T> = {
  [K in keyof T as K extends `on${infer Event}` ? Event : never]: T[K] extends (
    ...args: infer A
  ) => any
    ? A
    : never
}

interface ButtonProps {
  onClick: (e: MouseEvent) => void
  onHover: (e: MouseEvent) => void
  onFocus: (e: FocusEvent) => void
  disabled: boolean
  label: string
}

type ButtonEvents = ExtractEventHandlers<ButtonProps>
// {
//   Click: [e: MouseEvent]
//   Hover: [e: MouseEvent]
//   Focus: [e: FocusEvent]
// }

// 模拟 Vue 组件 props 推断
type ExtractProps<T> = T extends { props: infer P } ? P : {}

type ExtractEmits<T> = T extends { emits: infer E } ? E : {}

const component = {
  props: {
    title: String,
    count: Number,
  },
  emits: ['update', 'delete'],
}

type Props = ExtractProps<typeof component>
// { title: StringConstructor; count: NumberConstructor }

// 函数柯里化类型
type Curry<F> = F extends (...args: infer A) => infer R
  ? A extends [infer First, ...infer Rest]
    ? (arg: First) => Curry<(...args: Rest) => R>
    : R
  : never

function add(a: number, b: number, c: number): number {
  return a + b + c
}

type CurriedAdd = Curry<typeof add>
// (arg: number) => (arg: number) => (arg: number) => number

// ❌ 错误：infer 只能在条件类型的 extends 子句中使用
// type Wrong<T> = infer U;

// ✅ 正确
type Right<T> = T extends infer U ? U : never

// 同名 infer 会被推断为联合类型
type Foo<T> = T extends { a: infer U; b: infer U } ? U : never

type Result = Foo<{ a: string; b: number }>
// string | number

// 协变位置：联合
type Bar<T> = T extends { a: (x: infer U) => void; b: (x: infer U) => void }
  ? U
  : never

type Result2 = Bar<{ a: (x: string) => void; b: (x: number) => void }>
// string & number = never（逆变位置）

// 使用 const 断言保留字面量类型
function createConfig<T extends string>(key: T) {
  return { key }
}

const config = createConfig('api-key' as const)
// { key: 'api-key' }

// 在 infer 中推断字面量
type ExtractLiteral<T> = T extends `${infer L}` ? L : never

type Lit = ExtractLiteral<'hello'>
// 'hello'