当函数自己调用时，对象的方法可以在函数中调用，但是当通过 setTimeout 调用函数时，对象是未定义的

Question

我有以下 class 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;

我从另一个模块 server 中的 Express 端点调用 conference 函数，如下所示：

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();
});

调用 conference.announce 成功，控制台输出：

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

但是当调用 conference.setTimedAnnounce() 后 15 秒过去时，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调用 announce 时，它是以 urlSerializer 实例超出范围的方式进行的。当通过 setTimeout() 调用 announce 时，如何让它识别 urlSerializer？

编辑：我尝试按如下方式保存 this 范围：

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

但这给了我同样的错误。

编辑 2：

我使用了 Barmar 的 this.announce.bind(this) 解决方案并解决了 TypeError，但是参数数组和 URL 在 announce 的 setTimeout() 调用中没有正确构造；控制台输出如下：

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

我怀疑我需要在其他地方使用 .bind(this) 以便 urlSerializer.serialize() 会在适当的范围内用 this 被调用，但我还没有找到它还。

编辑 3：

没关系，我想通了——setTimeout 需要将 conferenceSid 作为参数传入以及 minutesRemaining。现在工作。

Answer 1

    setTimeout(this.announce,minutesToElapse*60000,minutesRemaining);

需要

    setTimeout(this.announce.bind(this),minutesToElapse*60000,minutesRemaining);

将函数属性作为参数传递不会绑定 this 上下文。它仅在您使用该语法调用方法时自动绑定。

当函数自己调用时，对象的方法可以在函数中调用，但是当通过 setTimeout 调用函数时，对象是未定义的

Object's method can be called in function when function is called on its own, but when function called via setTimeout the object is undefined

javascript

settimeout

node.js

es6-class