错误TypeError：无法在setInterval函数中读取未定义的属性'forEach'

Question

所以在下面的代码中，我在从websocket接收到一个对象之后推送它。在此之后，我想调用setInterval并每隔60秒向messages数组中对象的minutesAgo属性添加一分钟。

不幸的是，ERROR TypeError: Cannot read property 'forEach' of undefined函数中出现错误getMinutesAgo()。我之前将对象推送到messages数组中，所以我不明白为什么this.messages未定义。有人知道为什么吗？

export class MovieChatComponent implements OnInit{

    fullName;
    title = 'Websocket Demo';
    url;
    ws;
    messages = [];
    movieChatElem;

    constructor(private movieChatService: MovieChatService){

    }

    ngOnInit(){
        this.fullName = localStorage.getItem('fullName');
        this.url = 'ws://localhost:3185';
        this.movieChatService.createObservableSocket(this.url)
            .subscribe(obj => {
                console.log(obj);
                this.messages.push(obj);
                console.log(this.messages);
                this.movieChatElem.scrollTop = this.movieChatElem.scrollHeight - this.movieChatElem.clientHeight;
                this.startTimeCalculations()
                },
                err => console.log(err),
                () => console.log('The observable stream, is complete'));
    }

    startTimeCalculations(){
        setInterval(this.getMinutesAgo,60000);
    }

    getMinutesAgo (){
        console.log('entered');
        console.log(this.messages);
        this.messages.forEach(function (msg) {
            msg.minutesAgo++;
            console.log('done');
            console.log(msg.minutesAgo);
        })
    }
}