Question

所以我在尝试找出下面的代码片段时遇到了一些问题。截至目前，它正在for循环的'request（scanurl，.....'）部分运行之前返回值。循环确实在运行。而且我不认为'lines200'var正在通过更新柜台。我是一个学习者，所以任何解释将不胜感激。

async function processLineByLine(baseData) {
    console.log('enter async func')
try {
  const fileStream = fs.createReadStream('./file.txt');
  let linesTotal = 0;
  let lines200 = 0;

  const rl = readline.createInterface({
    input: fileStream
  });
  for await (let line of rl) {
    console.log('in loop')
    const scanurl = (baseData.match(/http/gi)) ? (baseData) : ('https://' + baseData + line);
    linesTotal++;

  request(scanurl, {json: true}, function (error, response, body) {
            let statusCode = response.statusCode;
            let htmlBody = body;
            //console.log(htmlBody)
            //console.log(statusCode)
            if (statusCode == "200") {
                console.log('in 2nd if')
                let $ = cheerio.load(htmlBody);
                let titleScrape = $('title').html();
                console.log(titleScrape)
                if (titleScrape.match(/404 | test/gi)) { 
                    console.log('Matched')
                } else {
                        lines200++;
                        console.log(lines200)
                    }
                } else {
                    // Do nothing
                }
            });

        }
        return {
            total: linesTotal,
            count200: lines200,
        }; 
} catch (error) {
    console.error(error)
}

}

router.get('/:reqTarget', async (req, res) => {
    console.log('starting')
    var baseUrl = req.params.reqTarget;
    try {
        console.log('in the try')
    const initTest = await processLineByLine(baseUrl);
    const {total, count200} = initTest;
    console.log(total, count200)
    if (initTest) return res.status(200).send({
        message: 'STATUS 200 COUNT: ' + count200 + ' ' + 'TOTAL: ' + total });

    } catch (error) {
        console.log(error)
    }

});

当前输出：

starting
in the try
enter async func
in loop
in loop
in loop
in loop
in loop
in loop
in loop
33 0 //this is the return that is two early
in 2nd if
404 | test
Matched
in 2nd if
404 | test
Matched

Answer 1

当您有一个包含异步操作的循环时，可以使用以下两个选项之一。您可以并行运行它们，并以某种方式跟踪它们何时完成。或者，您可以一个接一个地依次运行它们。看来您的循环可以用任何一种方式构造，但我将说明顺序选项。

async/await的出现使我们可以在中间用适当的for“暂停” await循环。但是，为此，所有异步操作都必须基于承诺，因此您可以await进行这些承诺。为此，我已从request()库切换到request-promise-native库，该库是使用本地内置Promise的request库的Promise包装。它还具有另一个不错的功能，它会自动检查2xx状态代码，因此您不必自己执行此操作。

这是该代码的样子：

const rp = require('request-promise-native');

async function processLineByLine(baseData) {
    console.log('enter async func')
    try {
        const fileStream = fs.createReadStream('./file.txt');
        let linesTotal = 0;
        let lines200 = 0;

        const rl = readline.createInterface({
            input: fileStream
        });
        for await (let line of rl) {
            console.log('in loop')
            const scanurl = (baseData.match(/http/gi)) ? (baseData) : ('https://' + baseData + line);
            linesTotal++;

            try {
                let htmlBody = await rp(scanurl, {json: true});
                let $ = cheerio.load(htmlBody);
                let titleScrape = $('title').html();
                console.log(titleScrape);
                if (titleScrape.match(/404 | test/gi)) {
                    console.log('Matched')
                } else {
                    lines200++;
                    console.log(lines200)
                }
            } catch(e) {
                console.log(`error on request(${scanurl})`, e);
                // like your original code, this will only log the error
                //   and then continue with the rest of the URLs
            }
        }
        return {
            total: linesTotal,
            count200: lines200,
        };
    } catch (error) {
        console.error(error)
    }

}

router.get('/:reqTarget', async (req, res) => {
    console.log('starting')
    var baseUrl = req.params.reqTarget;
    try {
        console.log('in the try')
        const initTest = await processLineByLine(baseUrl);
        const {total, count200} = initTest;
        console.log(total, count200)
        res.status(200).send({message: 'STATUS 200 COUNT: ' + count200 + ' ' + 'TOTAL: ' + total});

    } catch (error) {
        console.log(error)
        res.sendStatus(500);
    }

});

异步等待逻辑返回问题

1 个答案: