In-depth understanding of the use of the infer keyword in typescript

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

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

// promise response type type PromiseResType<T> = T extends Promise<infer R> ? R : T

// Verify 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>
// Inverse type StrPromiseRes = PromiseResType<StrPromise> // str

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

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]

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

type TTuple = [string, number];

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

// Get parameter type type ConstructorParameters<T extends new (...args: any[]) => any> = T extends new (...args: infer P) => any ? P : never;

// Get instance type 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

//Define function useReducer<R extends Reducer<any, any>, I>(
  reducer: R,
  // ReducerState inferred type initializerArg: I ​​& ReducerState<R>,
  initializer: (arg: I ​​& ReducerState<R>) => ReducerState<R>
): [ReducerState<R>, Dispatch<ReducerAction<R>>];

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


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

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']


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

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

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

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