javascript - Node JS 回调函数

Question

我最近正在深入研究 NodeJS(并使用 Nightmare.js)来解析网站，但在回调函数和显示返回结果方面遇到了问题。我试图在另一个函数中调用单独的函数，但似乎无法返回任何结果。它们都返回未定义。非常感谢对此的任何帮助。

 function getDetails(loadURL, callback){
    nightmare.goto(loadURL)
        .wait(2000)
        .evaluate(function(){
            var gigs = [];
            $('.hidden-xs .used-vehicle').each(function(){
                item = {}
                item["year"] = $(this).attr('data-year')
                item["make"] = $(this).attr('data-make')
                item["model"] = $(this).attr('data-model')
                item["body"] = $(this).attr('data-body')
                item["color"] = $(this).attr('data-ext-color')
                item["trim"] = $(this).attr('data-trim')
                item["mileage"] = $(this).attr('data-mileage')
                item["transmission"] = $(this).attr('data-transmission')
                item["vin"] = $(this).find(".vehicle-overview").attr('id')
                item["title"] = $(this).find(".vehicle-overview h2 a").text()
                item["link"] = $(this).find(".vehicle-overview h2 a").attr('href')
                item["price"] = $(this).find(".vehicle-content .price").text()
                gigs.push(item)
            })
            return gigs
        })
        .end()
        .then(function(result){
            var returnString = '';
            for(gig in result){
                returnString = returnString + result[gig].title + " " + result[gig].link + " " + result[gig].year + " " + result[gig].make + " " + result[gig].model + " " + result[gig].body + " " + result[gig].color + " " + result[gig].trim + " " + result[gig].transmission + " " + result[gig].vin + " " + result[gig].price + "\n"
            }
            callback(returnString)
        })  
}

    // We will need to get the total amount of pages that we need to parse
    function getInventory(sURL, callback){
        nightmare.goto(sURL)
            .wait(2000)
            .evaluate(function(){
                totals = [];
                items = {}
                totalCars = $('.total-found .count').text()
                carsOnPage = $('.hidden-xs .used-vehicle').size()
                items['carTotal'] = totalCars
                items['onPage'] = carsOnPage
                var pageCalc = (totalCars / carsOnPage)
                items['tPages'] = Math.ceil(pageCalc)
                totals.push(items)
                return totals
            })
            .end()
            .then(function(result){
                var totalCars = '';
                var totalPages = '';
                for (item in result){
                    totalPages = result[item].tPages
                    totalCars = result[item].carTotal               
                }
                counter = 0;
                newURL = '';
                returnDetails = '';
                for (i =0; i < totalPages; i++){
                    if (i == 0){
                        newURL = sURL;
                    } else {
                        counter = i + 1;
                        newURL = sURL + "#action=im_ajax_call&perform=get_results&_post_id=5&page=" + counter + "&show_all_filters=false";
                    }
                    //console.log(newURL)
                    getINV = getDetails(newURL, function(returnString){
                        callback(returnString)
                    })
                    returnDetails = returnDetails + getINV
                }
                callback(returnDetails)
            })
    }

    getInventory(startURL, function(result){
        console.log(result)
    })

Answer 1

我不会费心告诉你，你不应该将回调与这样的 promise 混合在一起。但现在让我们看看问题所在。

案例1

你也检查一下错误怎么样？也许您的脚本抛出错误。我可以看到您正在 .then 上调用回调，但在 .catch 上没有调用任何内容。也许那时永远不会获得任何数据。

案例2

让我们检查一下您的功能。您每次都在调用.end。您是否也每次都创建新的 Nightmare 实例？

在 getInventory 函数上，不应调用 .end。在 getDetails 函数上，您不应调用 .end。它正在结束 Nightmare 实例，而您正在丢失数据。

完成所有函数和工作后，调用 nightmare.end()。要正确执行此操作，您需要了解有关 Promise 检查下面案例 3 的更多信息。

案例3

了解 Promise 的工作原理。在下面的行中，您永远不会等待该函数完成。

getINV = getDetails(newURL, function(returnString){
 callback(returnString)
})

您应该等待 Promise 完成。另外，请确保 Nightmare 不会尝试同时浏览两个链接。

所以继续学习 Promise 和 async wait 的东西。

我该如何解决你的代码？

我会使用 Promise.all、.map 和一堆其他新东西。这里有一些为您完成的示例代码，不要复制粘贴或直接运行代码，尝试理解为什么它与您的代码不同以及它可能会产生什么结果。

const pLimit = require("promise-limit")(2);

function getDetails(loadURL) {
  return nightmare
    .goto(loadURL)
    .wait(2000)
    .evaluate(() => {
      const gigs = [];
      $(".hidden-xs .used-vehicle").each(function() {
        item = {};
        item["year"] = $(this).attr("data-year");
        item["make"] = $(this).attr("data-make");
        item["model"] = $(this).attr("data-model");
        item["body"] = $(this).attr("data-body");
        item["color"] = $(this).attr("data-ext-color");
        item["trim"] = $(this).attr("data-trim");
        item["mileage"] = $(this).attr("data-mileage");
        item["transmission"] = $(this).attr("data-transmission");
        item["vin"] = $(this)
          .find(".vehicle-overview")
          .attr("id");
        item["title"] = $(this)
          .find(".vehicle-overview h2 a")
          .text();
        item["link"] = $(this)
          .find(".vehicle-overview h2 a")
          .attr("href");
        item["price"] = $(this)
          .find(".vehicle-content .price")
          .text();
        gigs.push(item);
      });
      return gigs;
    })
    .then(result => {
      let returnString = "";
      for (gig in result) {
        returnString =
          `${returnString +
result[gig].title} ${result[gig].link} ${result[gig].year} ${result[gig].make} ${result[gig].model} ${result[gig].body} ${result[gig].color} ${result[gig].trim} ${result[gig].transmission} ${result[gig].vin} ${result[gig].price}\n`;
      }
      return returnString;
    })
    .catch(error => {
      throw new Error(error);
    });
}

// We will need to get the total amount of pages that we need to parse
function getInventory(sURL) {
  return nightmare
    .goto(sURL)
    .wait(2000)
    .evaluate(() => {
      totals = [];
      items = {};
      totalCars = $(".total-found .count").text();
      carsOnPage = $(".hidden-xs .used-vehicle").size();
      items["carTotal"] = totalCars;
      items["onPage"] = carsOnPage;
      const pageCalc = totalCars / carsOnPage;
      items["tPages"] = Math.ceil(pageCalc);
      totals.push(items);
      return totals;
    })
    .then(result => {
      let totalCars = "";
      let totalPages = "";
      for (item in result) {
        totalPages = result[item].tPages;
        totalCars = result[item].carTotal;
      }
      counter = 0;
      newURL = "";
      urls = [];
      returnDetails = [];
      for (i = 0; i < totalPages; i++) {
        if (i == 0) {
          newURL = sURL;
        } else {
          counter = i + 1;
          newURL =
            `${sURL}#action=im_ajax_call&perform=get_results&_post_id=5&page=${counter}&show_all_filters=false`;
        }
        // push to the url array
        // use .map for cleaner code
        urls.push(newURL);
      }
      // return a new promise with concurrency limit
      return Promise.all(
        urls.map(url => {
          return limit(() => getDetails(newURL));
        })
      );
    })
    .catch(error => {
      throw new Error(error);
    });
}

getInventory(startURL)
  .then(result => {
    console.log(result);
  })
  .catch(error => {
    console.err(error);
  });

资源:

• https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise
• https://javascript.info/async-await
• https://ponyfoo.com/articles/understanding-javascript-async-await