当函数本身被调用时,可以在函数中调用对象的方法,但当通过setTimeout调用函数时,对象未定义

我有下面的课 Conference :

//UrlSerializer is for encoding JSON objects into GET url
const UrlSerializer=require('./urlSerializer');

class Conference{

    constructor(client,workspace){
        this.client=client;
        this.workspace=workspace;
        this.urlSerializer=new UrlSerializer();
    }


    announce(conferenceSid,timeRemaining){
        var parameters={
            timeRemaining:timeRemaining
        }
        var url=this.urlSerializer.serialize('conferenceAnnounceTime',parameters);
        console.log("conference.announce url: "+url);
        this.client.conferences(conferenceSid)
            .update({
                announceUrl:url,
                announceMethod:'GET'
            })
            .then(conference=>console.log(conference.friendlyName));
    }

    setTimedAnnounce(initialMinutes,minutesToElapse,conferenceSid){
        var minutesRemaining=initialMinutes-minutesToElapse;
        setTimeout(this.announce,minutesToElapse*60000,minutesRemaining);
    }

}

module.exports=Conference;

UrlSerializer 是:

const querystring=require('querystring');
require('env2')('.env');

class UrlSerializer{

    constructor(){
        this.paramArrayName="parameters";
    }

    serialize(endpoint,paramArray){
        var url=process.env.APP_BASE_URL+"/"+endpoint;
        console.log("urlSerializer base url: "+url);
        var arrayString=JSON.stringify(paramArray);
        console.log("urlSerializer stringified parameter array: "+arrayString);
        var fullUrl=url+"?"+querystring.stringify({[this.paramArrayName]:arrayString});
        console.log("urlSerializer full url: "+fullUrl);
        return fullUrl;
    }   
}

module.exports=UrlSerializer;

我打电话 conference 来自不同模块中的Express端点的函数 server 如下:

app.get('/conferenceEvents',function(req,res){
    conferenceSid=req.query.ConferenceSid;
    conference.announce(conferenceSid,initialMinutes);
    conference.setTimedAnnounce(initialMinutes,0.25,conferenceSid);
    res.type('application/json');
    res.status(200).send();
});

呼唤 conference.announce 控制台输出成功:

urlSerializer base url: http://x.ngrok.io/conferenceAnnounceTime
urlSerializer stringified parameter array: {"timeRemaining":5}
urlSerializer full url: http://x.ngrok.io/conferenceAnnounceTime?parameters=%7B%22timeRemaining%22%3A5%7D
conference.announce url: http://x.ngrok.io/conferenceAnnounceTime?parameters=%7B%22timeRemaining%22%3A5%7D

但是当通话后15秒 conference.setTimedAnnounce() ,我得到以下错误:

TypeError: Cannot read property 'serialize' of undefined
    at Timeout.announce [as _onTimeout] (c:\thisAppPath\conference.js:45:30)
    at ontimeout (timers.js:502:15)
    at tryOnTimeout (timers.js:323:5)
    at Timer.listOnTimeout (timers.js:290:5)

我在想也许当计时器打电话的时候 announce 在某种程度上, urlSerializer 实例超出范围。我怎样才能认出它 URL序列化器 什么时候? 宣布 被称为通过 setTimeout() ?

编辑:我试图保存 this 范围如下:

    setTimedAnnounce(initialMinutes,minutesToElapse,conferenceSid){
        var that=this;
        var minutesRemaining=initialMinutes-minutesToElapse;
        setTimeout(that.announce,minutesToElapse*60000,minutesRemaining);
    }

但这也给了我同样的错误。

编辑2:

我用过Barmar的 this.announce.bind(this) 解决方案,解决了 TypeError ,但参数数组和url未在announce;console输出的setTimeout()调用中正确构造,如下所示:

urlSerializer base url: http://x.ngrok.io/conferenceAnnounceTime
urlSerializer stringified parameter array: {}
urlSerializer full url: http://x.ngrok.io/conferenceAnnounceTime?parameters=%7B%7D
conference.announce url: http://x.ngrok.io/conferenceAnnounceTime?parameters=%7B%7D

我怀疑还有别的地方需要我用 .bind(this) 以便 urlSerializer.serialize() 将被呼叫 这 在适当的范围内,但我还没有找到。

编辑3:

没关系,我知道了-- setTimeout 需要拥有 conferenceSid 作为参数传入 minutesRemaining . 现在开始工作。