typescript 中的 infer 關鍵字的理解

ESnail發表於2021-06-26
TypeScript

infer 這個關鍵字，整理記錄一下，避免後面忘記了。有點難以理解呢。

infer

infer 是在 typescript 2.8中新增的關鍵字。

infer 可以在 extends 條件型別的字句中，在真實分支中引用此推斷型別變數，推斷待推斷的型別。

例如：用infer推斷函式的返回值型別

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

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

在這個例子中，

T extends U ? X : Y的形式為條件型別。

infer R代表待推斷的返回值型別，如果T是一個函式(...args: any[]) => infer R，則返回函式的返回值R，否則返回any

案例：加深理解

反解 Promise

// promise 響應型別
type PromiseResType<T> = T extends Promise<infer R> ? R : T

// 驗證
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

反解函式入參型別

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 轉 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

// 獲取引數型別
type ConstructorParameters<T extends new (...args: any[]) => any> = T extends new (...args: infer P) => any ? P : never;

// 獲取例項型別
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

// 定義
function useReducer<R extends Reducer<any, any>, I>(
  reducer: R,
  // ReducerState 推斷型別
  initializerArg: I & ReducerState<R>,
  initializer: (arg: I & ReducerState<R>) => ReducerState<R>
): [ReducerState<R>, Dispatch<ReducerAction<R>>];

// infer推斷
type ReducerState<R extends Reducer<any, any>> = R extends Reducer<infer S, any>
  ? S
  : never;
// Reducer型別
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)
})

// 推斷出
const count: Ref<{
  foo: string;
  bar: number;
}>

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

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

參考

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