錯誤類型錯誤：無法讀取的不確定

Question

我使用lodash和angularfire2通過用戶數據的陣列試圖loop屬性「DB」，但我得到錯誤類型錯誤：無法讀取未定義

典型推動的特性「DB」陣列 [ 0：用戶1， 1：USER2， 2：用戶3， ]

  import { AngularFireDatabase} from 'angularfire2/database'; 
      import { AngularFireAuth } from 'angularfire2/auth'; 
      import * as firebase from 'firebase/app';; 
      import * as _ from 'lodash'; 

      constructor( 
       private db    : AngularFireDatabase, 
       private afAuth   : AngularFireAuth) {} 




      addGroupChats(allGroupMember) { 
      console.log("allGroupMember", allGroupMember); 

       //this line work well without error 
       console.log("value[0]", allGroupMember.users[0]); 
       //let endpoint = this.db.object(`/users/${allGroupMember.users[0]}/groups/${allGroupMember.groupUid}`); 
       //endpoint.set(true); 

       this line throws ERROR TypeError: Cannot read property 'db' of undefined 
       _.forEach(allGroupMember.users, function(value) { 
        let endpoint = this.db.object(`/users/${value}/groups/${allGroupMember.groupUid}`); 
        endpoint.set(true);  
       }); 
      } 

Answer 1

需要使用arrow functions回調。 this引用的是函數對象而不是您的案例中的類。 這裏：

_.forEach(allGroupMember.users,(value) => { 
        let endpoint = this.db.object(`/users/${value}/groups/${allGroupMember.groupUid}`); 
        endpoint.set(true);  
       })