如何在节点中按顺序从循环中返回数据

Question

我正在创建一个网络抓取工具，它会从该网站 (https://www.imdb.com/movies-coming-soon/) 抓取明年上映的所有电影，并循环访问一系列链接，其中包含明年每个月的所有电影，它可以工作，但唯一的问题是由于 node.js 异步行为，它没有按顺序返回它们，我如何让它循环遍历数组并按顺序返回数据？

我试着做一个回调函数，但我不知道它会去哪里

const request = require('request')
const cheerio = require('cheerio')

const movieArray = [ '/movies-coming-soon/2019-09/',
'/movies-coming-soon/2019-10/',
'/movies-coming-soon/2019-11/',
'/movies-coming-soon/2019-12/',
'/movies-coming-soon/2020-01/',
'/movies-coming-soon/2020-02/',
'/movies-coming-soon/2020-03/',
'/movies-coming-soon/2020-04/',
'/movies-coming-soon/2020-05/',
'/movies-coming-soon/2020-06/',
'/movies-coming-soon/2020-07/',
'/movies-coming-soon/2020-08/' ]
for (let i = 0; i < movieArray.length; i++) {
    request.get('https://www.imdb.com' + movieArray[i] , (err, res, body) => {
        if (!err && res.statusCode == 200) {
            console.log(res.request.href)
            const $ = cheerio.load(body)
            //console.log(next)
            $('h4').each((i, v) => {
                const date = $(v).text()
                console.log(date)
            })               
        }
    })
}

我希望它按顺序返回数据，而不是根据节点异步行为导致数据返回的速度按顺序返回

Answer 1

根据https://lavrton.com/javascript-loops-how-to-handle-async-await-6252dd3c795/ 的解释，这是 for 循环中的经典异步问题。以下是解决方案：

// const request = require('request')
const request = require('request-promise');
const cheerio = require('cheerio');

const movieArray = [
  '/movies-coming-soon/2019-09/',
  '/movies-coming-soon/2019-10/',
  '/movies-coming-soon/2019-11/',
  '/movies-coming-soon/2019-12/',
  '/movies-coming-soon/2020-01/',
  '/movies-coming-soon/2020-02/',
  '/movies-coming-soon/2020-03/',
  '/movies-coming-soon/2020-04/',
  '/movies-coming-soon/2020-05/',
  '/movies-coming-soon/2020-06/',
  '/movies-coming-soon/2020-07/',
  '/movies-coming-soon/2020-08/',
];

async function processMovieArray(array) {
  for (const item of array) {
    await getMovie(item);
  }
  console.log('Done');
}

async function getMovie(item) {
  const options = {
    method: `GET`,
    uri: 'https://www.imdb.com' + item,
  };
  const response = await request(options);
  const $ = cheerio.load(response.body);
  $('h4').each((i, v) => {
    const date = $(v).text();
    console.log(date);
  });
}

processMovieArray(movieArray);

Answer 2

与当前代码偏差最小的低技术方法是仅使用 for 循环的索引来填充数组。由于for 循环中的let 将为i 循环的每次迭代创建一个单独的变量for，因此我们可以在异步回调中使用该索引来引用结果数组中的所需位置。然后，您还可以使用cntr 来了解您何时完成所有结果：

const request = require('request');
const cheerio = require('cheerio');

if (!Array.prototype.flat) {
    Array.prototype.flat = function() {
        return this.reduce((acc, val) => acc.concat(val), []);
    }
}


const movieArray = [ '/movies-coming-soon/2019-09/',
'/movies-coming-soon/2019-10/',
'/movies-coming-soon/2019-11/',
'/movies-coming-soon/2019-12/',
'/movies-coming-soon/2020-01/',
'/movies-coming-soon/2020-02/',
'/movies-coming-soon/2020-03/',
'/movies-coming-soon/2020-04/',
'/movies-coming-soon/2020-05/',
'/movies-coming-soon/2020-06/',
'/movies-coming-soon/2020-07/',
'/movies-coming-soon/2020-08/' ];

let results = [];
let cntr = 0;
for (let i = 0; i < movieArray.length; i++) {
    request.get('https://www.imdb.com' + movieArray[i] , (err, res, body) => {
        ++cntr;
        if (!err && res.statusCode == 200) {
            console.log(res.request.href)
            const $ = cheerio.load(body)
            //console.log(next)
            let textArray = [];
            $('h4').each((i, v) => {
                console.log(date)
                textArray.push($(v).text());
            });
            results[i] = textArray;
        }
        if (cntr === moveArray.length) {
            // all results are done now
            let allResults = results.flat();
        }
    })
}

更优雅一点的方法是切换到 Promise 并让 Promise 基础架构为您保持一切井井有条：

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

if (!Array.prototype.flat) {
    Array.prototype.flat = function() {
        return this.reduce((acc, val) => acc.concat(val), []);
    }
}

const movieArray = [ '/movies-coming-soon/2019-09/',
'/movies-coming-soon/2019-10/',
'/movies-coming-soon/2019-11/',
'/movies-coming-soon/2019-12/',
'/movies-coming-soon/2020-01/',
'/movies-coming-soon/2020-02/',
'/movies-coming-soon/2020-03/',
'/movies-coming-soon/2020-04/',
'/movies-coming-soon/2020-05/',
'/movies-coming-soon/2020-06/',
'/movies-coming-soon/2020-07/',
'/movies-coming-soon/2020-08/' ];

// 
if (!Array.prototype.flat) {
    Array.prototype.flat = function() {
        return this.reduce((acc, val) => acc.concat(val), []);
    }
}

Promise.all(movieArray.map(path => {
    return rp('https://www.imdb.com' + path).then(body => {
        const $ = cheerio.load(body);
        let textArray = [];
        $('h4').each((i, v) => {
            // console.log($(v).text());
            textArray.push($(v).text());
        });
        return textArray;

    }).catch(err => {
        // ignore errors on urls that didn't work
        // so we can get the rest of the results without aborting
        console.log("err");
        return undefined;
    });
})).then(results => {
    // flatten the two level array and remove empty items
    let allResults = results.flat().filter(item => !!item);
    console.log(allResults);
}).catch(err => {
    console.log(err);
});

仅供参考，我在 nodejs 版本 10.16.0 中测试了第二个版本，它可以工作。