infer 这个关键字，整理记录一下，避免后面忘记了。有点难以理解呢。

infer

infer 是在 typescript 2.8中新增的关键字。

infer 可以在 extends 条件类型的字句中，在真实分支中引用此推断类型变量，推断待推断的类型。

例如：用infer推断函数的返回值类型

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

type fn = () => number
type fnReturnType = ReturnType<fn> // number

在这个例子中，

T extends U ? X : Y的形式为条件类型。

infer R代表待推断的返回值类型，如果T是一个函数(...args: any[]) => infer R，则返回函数的返回值R，否则返回any

案例：加深理解

反解 Promise

// promise 响应类型
type PromiseResType<T> = T extends Promise<infer R> ? R : T

// 验证
async function strPromise() {
  return 'string promise'
}

interface Person {
  name: string;
  age: number;
}
async function personPromise() {
  return {
    name: 'p',
    age: 12
  } as Person
}

type StrPromise = ReturnType<typeof strPromise> // Promise<string>
// 反解
type StrPromiseRes = PromiseResType<StrPromise> // str

type PersonPromise = ReturnType<typeof personPromise> // Promise<Person>
// 反解
type PersonPromiseRes = PromiseResType<PersonPromise> // Person

反解函数入参类型

type Fn<A extends any[]> = (...args: A) => any
type FnArgs<T> = T extends Fn<infer A> ? A : any

function strFn (name: string) {

}

type StrFn = FnArgs<typeof strFn> // [string]

tuple 转 union ，如：[string, number] -> string | number

type ElementOf<T> = T extends Array<infer E> ? E : never

type TTuple = [string, number];

type ToUnion = ElementOf<ATuple>; // string | number

new 操作符

// 获取参数类型
type ConstructorParameters<T extends new (...args: any[]) => any> = T extends new (...args: infer P) => any ? P : never;

// 获取实例类型
type InstanceType<T extends new (...args: any[]) => any> = T extends new (...args: any[]) => infer R ? R : any;

class TestClass {

  constructor(
    public name: string,
    public string: number
  ) {}
}

type Params = ConstructorParameters<typeof TestClass>;  // [string, numbder]

type Instance = InstanceType<typeof TestClass>;         // TestClass

react - reducer

// 定义
function useReducer<R extends Reducer<any, any>, I>(
  reducer: R,
  // ReducerState 推断类型
  initializerArg: I & ReducerState<R>,
  initializer: (arg: I & ReducerState<R>) => ReducerState<R>
): [ReducerState<R>, Dispatch<ReducerAction<R>>];

// infer推断
type ReducerState<R extends Reducer<any, any>> = R extends Reducer<infer S, any>
  ? S
  : never;
// Reducer类型
type Reducer<S, A> = (prevState: S, action: A) => S;


// 使用 reducer
const reducer = (x: number) => x + 1;
const [state, dispatch] = useReducer(reducer, '');
// Argument of type "" is not assignable to parameter of type 'number'.

vue3 - ref

export interface Ref<T = any> {
  [isRefSymbol]: true
  value: T
}

export function ref<T>(value: T): T extends Ref ? T : Ref<UnwrapRef<T>>

export type UnwrapRef<T> = {
  cRef: T extends ComputedRef<infer V> ? UnwrapRef<V> : T
  ref: T extends Ref<infer V> ? UnwrapRef<V> : T
  array: T
  object: { [K in keyof T]: UnwrapRef<T[K]> }
}[T extends ComputedRef<any>
  ? 'cRef'
  : T extends Array<any>
    ? 'array'
    : T extends Ref | Function | CollectionTypes | BaseTypes
      ? 'ref' // bail out on types that shouldn't be unwrapped
      : T extends object ? 'object' : 'ref']


// 使用
const count = ref({
  foo: ref('1'),
  bar: ref(2)
})

// 推断出
const count: Ref<{
  foo: string;
  bar: number;
}>

const count = ref(2) // Ref<number>

const count = ref(ref(2)) // Ref<number>

参考

• 理解TypeScript中的infer关键字
• Vue3 跟着尤雨溪学 TypeScript 之 Ref 类型从零实现
• 巧用 TypeScript（五）---- infer