Node.js导出导入失效：爬虫进度值无法同步问题求助

我在Web爬虫的导出与app.js的导入环节遇到问题。从ticker函数中调用percentage函数时，无法通过返回值传递数据，希望更新roundedpercent变量并导出当前值，但app.js中始终输出0，即便爬虫已启动且进度不再为0%。

我的代码如下： #

trader-german.js

async function starttrger(){
//...
    function ticker() {
        if (scraped == lastIndex) {
            percentage(scraped, lastIndex);
            //...
        } else {
            percentage(scraped, lastIndex);
    }
}

var percent = 0;
var roundedpercent = 0;
function percentage(scraped, lastIndex){
    percent = scraped * 100 / lastIndex
    roundedpercent = Math.round(percent)
    console.log(roundedpercent) // 此处输出正确
}

module.exports = {
    starttrger,
    roundedpercent
}

app.js

const { starttrger,roundedpercent} = require('./trader-german');
app.post('/starttrger', async(req, res) => {
    await starttrger();
    res.send({ message: 'Webscraper for trader-german started'});
});
function logProgress() {
    console.log(`current Progress: ${roundedpercent}%`);
}
setInterval(logProgress, 1000);

trader-german.js中的console.log能正确输出当前百分比，但app.js中每次都输出0，求解决方案。

问题原因 #

你导出的roundedpercent是原始数值类型变量，JavaScript中基本类型是值传递——app.js导入的只是初始值0的副本，trader-german.js后续对该变量的更新不会同步到副本上，所以始终显示0。

解决方案 #

方法一：用对象包裹进度变量

利用对象的引用传递特性，让主文件和爬虫文件共享同一个对象引用：

修改trader-german.js：

async function starttrger(){
//...
    function ticker() {
        if (scraped == lastIndex) {
            percentage(scraped, lastIndex);
            //...
        } else {
            percentage(scraped, lastIndex);
    }
}

// 用对象包裹进度变量
const progress = {
    roundedpercent: 0
};

function percentage(scraped, lastIndex){
    const percent = scraped * 100 / lastIndex
    progress.roundedpercent = Math.round(percent)
    console.log(progress.roundedpercent)
}

module.exports = {
    starttrger,
    progress
}

修改app.js：

const { starttrger, progress} = require('./trader-german');
app.post('/starttrger', async(req, res) => {
    await starttrger();
    res.send({ message: 'Webscraper for trader-german started'});
});
function logProgress() {
    console.log(`current Progress: ${progress.roundedpercent}%`);
}
setInterval(logProgress, 1000);

方法二：提供获取进度的函数

通过函数调用实时获取最新的进度值：

修改trader-german.js：

async function starttrger(){
//...
    function ticker() {
        if (scraped == lastIndex) {
            percentage(scraped, lastIndex);
            //...
        } else {
            percentage(scraped, lastIndex);
    }
}

let roundedpercent = 0;
function percentage(scraped, lastIndex){
    const percent = scraped * 100 / lastIndex
    roundedpercent = Math.round(percent)
    console.log(roundedpercent)
}

// 新增获取进度的函数
function getProgress() {
    return roundedpercent;
}

module.exports = {
    starttrger,
    getProgress
}

修改app.js：

const { starttrger, getProgress} = require('./trader-german');
app.post('/starttrger', async(req, res) => {
    await starttrger();
    res.send({ message: 'Webscraper for trader-german started'});
});
function logProgress() {
    console.log(`current Progress: ${getProgress()}%`);
}
setInterval(logProgress, 1000);

方法三：用事件发射器主动通知进度（适合复杂场景）

无需轮询，进度更新时主动触发事件通知主文件：

修改trader-german.js：

const EventEmitter = require('events');
const progressEmitter = new EventEmitter();

async function starttrger(){
//...
    function ticker() {
        if (scraped == lastIndex) {
            const currentProgress = percentage(scraped, lastIndex);
            // 触发进度更新事件
            progressEmitter.emit('progress-update', currentProgress);
            //...
        } else {
            const currentProgress = percentage(scraped, lastIndex);
            progressEmitter.emit('progress-update', currentProgress);
    }
}

function percentage(scraped, lastIndex){
    const percent = scraped * 100 / lastIndex
    const roundedpercent = Math.round(percent)
    console.log(roundedpercent)
    return roundedpercent;
}

module.exports = {
    starttrger,
    progressEmitter
}

修改app.js：

const { starttrger, progressEmitter} = require('./trader-german');
app.post('/starttrger', async(req, res) => {
    await starttrger();
    res.send({ message: 'Webscraper for trader-german started'});
});

// 监听进度更新事件
progressEmitter.on('progress-update', (progress) => {
    console.log(`current Progress: ${progress}%`);
});

内容的提问来源于stack exchange，提问作者Hannes Z