Javascript core.js:6150 ERROR-TypeError:this.object在角度上不可编辑

我想在HTML文件中显示用户的名字

  getFirstName(id: any){
    this.users = this.afs.collection('users', ref => ref.where("uid", "==", id)).valueChanges();

    this.users.subscribe(users => {
      this.users = users;
    })
    let fname;
    for(let user of this.users){
        fname = user.firstName;
    }
    return fname;
  }

但当名字显示在HTML中时，编译器会出错

core.js:6150 ERROR TypeError: this.users is not iterable

---

不要使用与订阅对象和数据相同的变量this.users。您还需要在订阅中使用for of

fname  = null;

  getFirstName(id: any){
    this.users = this.afs.collection('users', ref => ref.where("uid", "==", id)).valueChanges();

    this.users.subscribe(users => {
      for(let user of users){
        this.fname = user.firstName;
      }
    })
  }