[TypeScript][Angular]メソッド(関数)の引数の中に関数？助けてください。

下記コードのupdateメソッドの仮引数(fn: (state: State) => State)とはどういう意味でしょうか？
このメソッドが呼び出されたとき引数には何が入ってくるのでしょうか？

コードを読んでいますが動きが読めません。助けてください

store.service.ts

tsx

import { Injectable } from '@angular/core'; import { BehaviorSubject, queueScheduler } from 'rxjs';import { map, distinctUntilChanged } from 'rxjs/operators';import { State, initialState } from './state'; @Injectable({ providedIn: 'root' })export class Store { private _state$ = new BehaviorSubject<State>(initialState); update(fn: (state: State) => State) { const current = this._state$.value; queueScheduler.schedule(() => { this._state$.next(fn(current)); }); } select<T>(selector: (state: State) => T) { return this._state$ .pipe( map(selector), distinctUntilChanged(), ); }}

user-detail.usecase.ts

tsx

import { Injectable } from '@angular/core';import { Store } from './store.service';import { UserApiService } from './user-api.service'; @Injectable({ providedIn: 'root' })export class UserDetailUsecase { get user$() { return this.store.select(state => state.userDetail.user); } constructor(private userApi: UserApiService, private store: Store) {} async fetchUser(userId: string) { this.store.update(state => ({ ...state, userDetail: { ...state.userDetail, user: null } })); const user = await this.userApi.getUserById(userId); this.store.update(state => ({ ...state, userDetail: { ...state.userDetail, user } })); }}

tsx

import { User } from './user'; export interface UserListFilter { nameFilter: string;} export interface State { userList: { items: User[]; filter: UserListFilter }, userDetail: { user: User | null; }} export const initialState = { userList: { items: [], filter: { nameFilter: '', } }, userDetail: { user: null }};

よろしくお願いいたします。