对象文字可能只指定已知属性

我正在研究如何在youtube视频系列之后创建Angular Firebase聊天：https://www.youtube.com/watch?v=-j0LVc-zLh0

在这里，我正在开发一个聊天服务（chat.service.ts），如下所示：

import {
    Injectable
} from '@angular/core';
import {
    AngularFireList, AngularFireDatabase
} from 'angularfire2/database';
import {
    AngularFireAuth
} from 'angularfire2/auth';
import {
    Observable
} from 'rxjs/Observable';
import {
    ChatMessage
} from '../models/chat-message.model';
import {
    AuthService
} from '../services/auth.service';
import * as firebase from 'firebase/app';



@Injectable()
export class ChatService {
    user: any;
    chatMessages: AngularFireList < ChatMessage[]> ;
    chatMessage: ChatMessage;
    userName: Observable<string>;

    constructor(
        private db: AngularFireDatabase,
        private angularFireAuth: AngularFireAuth
    ) {
        // this.angularFireAuth.authState.subscribe(auth => {
        //  if (auth !== undefined && auth !== null) {
        //      this.user = auth;
        //  }
        // })

    }

    sendMessage(msg: string) {
        const timestamp = this.getTimestamp();
        // const email = this.user.email;
        const email = "ex@ex.com";
        this.chatMessages = this.getMessages();
        this.chatMessages.push({
            message: msg,
            timeSent: timestamp,
            userName: this.userName,
            email: email
        }); 
    }

    getMessages(): AngularFireList<ChatMessage[]> {
        return this.db.list('messages', ref => ref.orderByKey().limitToLast(25)
        // console.log("test ok")
    )};

    getTimestamp() {
        const now = new Date();
        const date = now.getUTCFullYear() + '/' + (now.getUTCMonth() + 1) + '/' + now.getUTCDate();
        const time = now.getUTCHours() + ':' + (now.getUTCMinutes()) + ':' + now.getUTCSeconds();
        return (date + ' ' + time);
    }
}

在sendMessage函数中，当我尝试将消息输入ChatMessages []时， message: msg 行被标记为错误，我在终端中收到以下错误消息 .

src / app / services / chat.service.ts（48,4）中的错误：错误TS2345：类型'{message：string; timeSent：string; userName：Observable;电子邮件：string; }'不能分配给'ChatMessage []'类型的参数 . 对象文字只能指定已知属性，'ChatMessage []'类型中不存在'message' .

有人可以帮我这个吗？