TypeScript传入LogChanges的回调函数调用返回undefined如何解决

已知错误触发逻辑但未定位到具体修复点，涉及RxJS subscribe 嵌套调用的业务代码如下：

.subscribe(([a, b]: [any, any]) => {
  if (this.files.length > 0) {
    const setImagesStatus = (b.body.objectData as DtoSetAppBackgroundImagesStatus);
    if (setImagesStatus.success) {
      this.hasBackgroundImages = true;
      this.files = [];
      notify('Data successfully updated', 'success');
    } else {
      notify('Application background images upload error. ' + setImagesStatus.info, 'error', 9000);
    }
  } else {
    notify('Data successfully updated', 'success');
  }
  const parsedUserBeforeChanges = new UserPreLogDataConverter(this.groupsService,this.groups).GetPreLogData(updateUserDto);
  const parsedUserAfterChanges = new UserPreLogDataConverter(this.groupsService,this.groups).GetPreLogData(this.userWithNoChanges);
  this.eventLogEndpointSvc
    .addLogs(this.serverEventLogger
      .LogChanges(JSON.parse(JSON.stringify(parsedUserBeforeChanges)),JSON.parse(JSON.stringify(parsedUserAfterChanges)),
  () => {
    this.userCustomLogFunction(parsedUserBeforeChanges,parsedUserAfterChanges);
  }
  )).subscribe().add(this.userWithNoChanges=JSON.parse(JSON.stringify(updateUserDto)));}
);

this.userCustomLogFunction 方法实现如下：

userCustomLogFunction(userWithNoChanges: UserPreLogData,updateUserDto: UserPreLogData): Array<DtoEventLog>
{
  const oldGroupIds = userWithNoChanges.groups.map(x=>x.id);
  const newGroupIds = updateUserDto.groups.map(x=>x.id);

  const addedToGroups = newGroupIds.filter(x => !oldGroupIds.some(y => JSON.stringify(x) === JSON.stringify(y)));
  const deletedFromGroups = oldGroupIds.filter(x => !newGroupIds.some(y => JSON.stringify(x) === JSON.stringify(y)));

  const groupLogs:  DtoEventLog[] = [];
  for(const id in addedToGroups)
  {
      const evLog = new DtoEventLog();
      evLog.methodName = 'LiftaKitMobileAppTestUsers.Update';
      evLog.objectId=addedToGroups[id].toString();
      evLog.login=this.authService.getUser().login;
      evLog.firstName =this.authService.getUser().firstName;
      evLog.lastName = this.authService.getUser().lastName;
      evLog.email = this.authService.getUser().email;
      evLog.logType=EventLogType.UPDATE.toString();
      evLog.scope=EventLogScope.GROUP.toString();
      evLog.input ='Added user' + updateUserDto.firstName + ' ' + updateUserDto.lastName + ' login: ' + updateUserDto.login + ' no: '+ updateUserDto.no +', id: ' + updateUserDto.iD;
      groupLogs.push(evLog);
  }

  for(const id in deletedFromGroups)
  {
      const evLog = new DtoEventLog();
      evLog.methodName = 'LiftaKitMobileAppTestUsers.Update';
      evLog.objectId=deletedFromGroups[id].toString();
      evLog.login=this.authService.getUser().login;
      evLog.firstName =this.authService.getUser().firstName;
      evLog.lastName = this.authService.getUser().lastName;
      evLog.email = this.authService.getUser().email;
      evLog.logType=EventLogType.UPDATE.toString();
      evLog.scope=EventLogScope.GROUP.toString();
      evLog.input ='Deleted user' + updateUserDto.firstName + ' ' + updateUserDto.lastName + ' login: ' + updateUserDto.login + ' no: '+ updateUserDto.no +', id: ' + updateUserDto.iD;
      groupLogs.push(evLog);
  }

  return groupLogs;
}

代码整体运行无其他报错，仅存在异常：在LogChanges方法内部通过logMethod()调用传入的回调方法时，始终返回undefined，初步怀疑第一层subscribe中addLogs和LogChanges的调用写法存在错误。

LogChanges 方法实现如下：

LogChanges<Type>(ob1: Type,ob2: Type=null,logMethod: Function = () => new Array<DtoEventLog>()): Array<DtoEventLog> {
const additionalRelation = logMethod();
//rest code here
 }

和RxJS subscribe嵌套写法无关，问题出在传入LogChanges的回调函数本身：
你传入的是带大括号的代码块形式箭头函数() => { 调用方法 }，这类箭头函数不会自动返回内部执行结果，没有手动写return的情况下，函数执行完默认返回undefined，所以LogChanges里拿到的返回值永远是undefined。

两种改法任选其一即可：

• 显式返回方法执行结果，修改传入的回调函数：

() => {
  return this.userCustomLogFunction(parsedUserBeforeChanges,parsedUserAfterChanges);
}

• 用箭头函数隐式返回写法，去掉大括号，单表达式会自动返回执行结果：

() => this.userCustomLogFunction(parsedUserBeforeChanges,parsedUserAfterChanges)

内容的提问来源于stack exchange，提问作者The Trainer