infer

infer 是在 typescript 2.8中新增的关键字。

infer 能够在 extends 条件类型的字句中，在实在分支中援用此推断类型变量，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 = () => numbertype fnReturnType = ReturnType<fn> // number

在这个例子中，

T extends U ? X : Y的模式为条件类型。

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：加深了解

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> // strtype 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) => anytype FnArgs<T> = T extends Fn<infer A> ? A : anyfunction 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 : nevertype 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;// 应用 reducerconst 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']// 应用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

  • 了解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