自定义 Object 返回错误：类型“void”上不存在属性“forEach”.ts(2339)

Question

I have a variable that will either be of a class User or void , the only problem is that when I run this code it gives me an error of: Property 'forEach' does not exist on type 'void'.ts(2339) . I have tried to work around this a hundred different ways, but nothing works.

let friends: [User] | void = await this.getFriendsPosts(host);
friends.forEach(element => {

});

Update:

Here is my code for the function this.getFriendsPosts(host) and other subsequent functions:

async getFriendsPosts(host: User){
 let userFriendsID = host.friends
 let userFriends = []
 let retrievedPosts: [Post[]?] = []
 userFriendsID.map(async uid =>{
    let val = new User(await this.checkFriend(uid))
    userFriends.push(val)
    console.log(userFriends)
  })
 }


//*convert to users with promises
async checkFriend(uid){
  let metaData;
  let postInterface= [];
  let friendInterface : unknown;
  //*getting the post history
  let promise1 = new Promise((res, rej) => {
    metaData = 
this.firestore.collection('Users').doc(uid.toString())
    let postData = metaData.collection('Posts')
    postData.valueChanges().subscribe(vals =>{
      vals.forEach(input => {
        postInterface.push({
          title: input.title,
          id: input.id,
          user: input.user,
          timebomb: input.timebomb,
          type: input.type,
          timestamp: input.timestamp
        } as complexPostInterface)      
      })
      res(vals)
    })
  })
  //*getting the user stats
  let promise2 = new Promise((res, rej) => {
    metaData.valueChanges().subscribe(vals =>{
      friendInterface = {
        name: vals.name,
        uid: vals.uid,
        nickname: vals.nickname,
        currentPosts: postInterface,
        status: new Post(postInterface[0] as any),
        timestamp: vals.timestamp,
      } as complexUserObj
      res(friendInterface as complexUserObj)
    })
  })
  await Promise.all([promise1,promise2])
  console.log(friendInterface)
  return (friendInterface)
}

Answer 1

Your getFriendsPosts method should return undefined instead of void. Void should be used in functions whose return value will be ignored.

Property 'forEach' does not exist on undefined either, so you'll need to check first that friends exist:

let friends: [User] | undefined = await this.getFriendsPosts(host);
if (friends) {
  friends.forEach(element => {

  });
}

Or alternatively make the method return an empty array of Users and you don't need to check for it.