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>