Javascript 回调函数节点JS

我最近潜入nodejs，使用噩梦.js解析一个网站，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一个单独的函数，但似乎无法返回任何结果。它们都返回未定义。在此方面的任何帮助都将不胜感激

 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)
    })

---

尝试使用回调而不是返回

function getDetails(loadURL, callback){
    nightmare.goto(loadURL)
        .wait(2000)
        .evaluate(function(callback){
            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)
            })
            callback(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)
    })

---

也许下面的方法会起作用，将循环更改为reduce和map，去掉jQuery并进行一些小的更改

最重要的是：

从html元素获取属性会返回一个字符串，您应该将其转换为数字。 摆脱回调，让函数返回承诺。 代码如下：

const getDetails = loadURL =>
nightmare.goto(loadURL)//return promise here
.wait(2000)
.evaluate(
  ()=>
    Array.from(document.querySelectorAll('.hidden-xs .used-vehicle'))
    .reduce(
      (all,item)=>
        all.concat(
          [
            element.getAttribute('data-year'),
            element.getAttribute('data-make'),
            element.getAttribute('data-model'),
            element.getAttribute('data-body'),
            element.getAttribute('data-ext-color'),
            element.getAttribute('data-trim'),
            element.getAttribute('data-mileage'),
            element.getAttribute('data-transmission'),
            element.querySelector(".vehicle-overview").getAttribute('id'),
            element.querySelector(".vehicle-overview h2 a").innerText,
            element.querySelector(".vehicle-overview h2 a").getAttribute('href'),
            element.querySelector(".vehicle-content .price").innerText
          ].join(" ")
        ),
      []//the all array
    )
);

// We will need to get the total amount of pages that we need to parse
const getInventory = sURL =>
nightmare.goto(sURL)
  .wait(2000)
  .evaluate(
    ()=> {
      //there is only one item here, not sure why you push it into totals
      //  and call it items
      const item = {}
      //getAttribute returns a string, parse it to number
      totalCars = parseInt(document.querySelector('.total-found .count').innerText,10);
      carsOnPage = document.querySelectorAll('.hidden-xs .used-vehicle').length;
      item['carTotal'] = totalCars
      item['onPage'] = carsOnPage
      var pageCalc = (totalCars / carsOnPage)
      item['tPages'] = Math.ceil(pageCalc)
      return item;  
    }
  )
  .then(
    totalItem =>{
      var totalCars = '';
      var totalPages = '';
      totalPages = totalItem.tPages
      totalCars = totalItem.carTotal
      newURL = '';
      returnDetails = '';
      return Array.from(new Array(totalPages),(_,index)=>index+1)
      .reduce(
        (p,counter)=>
          p.then(
            results=>{
              if (counter === 1) {
                newURL = sURL;
              } else {
                newURL = sURL + "#action=im_ajax_call&perform=get_results&_post_id=5&page=" + counter + "&show_all_filters=false";
              }
              return getDetails(newURL)
              .then(
                result=>results.concat(result)
              );
            }
          ),
          Promise.resolve([])
      );
    }
  );

getInventory(startURL)
.then(
result=>
  console.log(result)
).catch(
err=>
  console.warn("Something went wrong:",err)
);

---

我不想麻烦告诉你，你不应该把回电和这样的承诺混为一谈。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？也许你的脚本出错了。我可以看到你正在打电话回叫。然后什么也没打。接住。也许那时他从未得到任何数据

案例2 让我们检查一下你的函数。你在呼叫。每次都结束。你每次也在创建新的噩梦实例吗

在getInventory函数中，不应调用.end。在getDetails函数中，不应调用.end。它正在结束噩梦般的实例，而您正在丢失数据

完成所有函数和工作后，调用噩梦.end。要正确地做到这一点，您需要了解下面的承诺检查案例3的更多信息

案例3 了解承诺是如何运作的。在下面的一行中，您永远不会等待函数完成

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

你应该等待承诺完成。另外，请确保噩梦不会试图同时浏览两个链接

因此，请继续学习承诺和异步等待的内容

我将如何解决您的代码

我会使用Promise.all、.map和其他一些新东西。下面是一些为您编写的示例代码，不要复制粘贴或直接运行代码，请尝试理解为什么它与您的代码不同，以及它会产生什么结果

常数pLimit=requirepromise-limit2； 函数getDetailsloadURL{ 回归噩梦 戈托拉杜尔先生 wait2000先生 .evaluate=>{ 常量gigs=[]； $.hidden xs.used-vehicle.eachfunction{ 项目={}； 项目[年度]=$this.attrdata-year； item[make]=$this.attrdata-make； 项目[模型]=$this.attrdata-model； item[body]=$this.attrdata-body； 项目[颜色]=$this.attrdat-ext-color； item[trim]=$this.attrdata-trim； 项目[里程]=$this.attrdata-里程； 项目[传输]=$this.attr数据传输； 项目[vin]=$this .查找.车辆概述 .attrid； 项目[标题]=$this .查找.车辆概述h2 a 文本 项目[链接]=$this .查找.车辆概述h2 a .attrref； 项目[价格]=$this .find.vehicle-content.price 文本 gigs.pushitem； }; 返回演出； } .thenresult=>{ 让returnString=； 在结果中{ 返回字符串= `${returnString+ 结果[gig]。标题}${result[gig]。链接}${result[gig]。年份}${result[gig]。制作}${result[gig]。模型}${result[gig]。正文}${result[gig]。颜色}${result[gig]。修剪}${result[gig]。传输}${result[gig]。vin}${result[gig]。价格}\n`； } 返回字符串； } .catcherror=>{ 抛出新的错误； }; } //我们需要得到需要解析的页面总数 函数getInventorysURL{ 回归噩梦 gotosURL先生 wait2000先生 .evaluate=>{ 总数=[]； 项目={}； totalCars=$.total found.count.text； carsOnPage=$.hidden xs.used-vehicle.size； 项目[carTotal]=车辆总数； 项目[第页]=汽车第页； const pageCalc=总汽车/汽车页面； 项目[tPages]=Math.ceilpageCalc； 总计6个项目； 返回总数； } .thenresult=>{ 让我们来看看汽车=； 设totalPages=； 对于结果中的项目{ totalPages=结果[项目].tPages； totalCars=结果[项目].carTotal； } 计数器=0； 新网址=； URL=[]； returnDetails=[]； 对于i=0；i{ 返回限制=>getDetailsnewURL； } ; } .catcherror=>{ 抛出新的错误； }; } getInventorystartURL .thenresult=>{ console.logresult； } .catcherror=>{ console.errerror；

---

}; 学习异步函数的承诺和回调是一项相当艰巨的任务。我能够让这项工作与以下。谢谢你们的帮助和指导。每个答案都让我陷入了一个不同的困境，最终找到了解决办法

    function getInventory(sURL){    
    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
        })
        .then(result => {
            var totalCars = '';
            var totalPages = '';
            for (item in result){
                totalPages = result[item].tPages
                totalCars = result[item].carTotal               
            }
            counter = 0;
            let links = [];
            let returnLinks = '';
            newURL = '';            
            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";
                }
                links.push(newURL);
            }
            return links;
        })
        .then(results => {
            var arrayLinks = results;
            arrayLinks.reduce(function(accumulator, url){
                return accumulator.then(function(newResults){
                    return nightmare.goto(url)
                        .wait(5000)
                        .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"] = $(this).find(".vehicle-content .price").text();
                                gigs.push(item);
                            });
                            return gigs;
                        })
                        .then(detail => {                       
                            for (gig in detail) {
                                try {
                                    var carVin = detail[gig].vin;
                                    var carTitle = detail[gig].title;
                                    var carDescrip = detail[gig].year + " " + detail[gig].make + " " + detail[gig].model;
                                    var carURL = detail[gig].link;
                                    var carMake = detail[gig].make;
                                    var carModel = detail[gig].model;
                                    var carYear = detail[gig].year;
                                    var carMileageFull = detail[gig].mileage;
                                    var carMileage = carMileageFull.replace(',', '');
                                    var carTransmission = detail[gig].transmission;
                                    var carBody = detail[gig].body;
                                    var carPriceFull = detail[gig].price;
                                    var carPriceFull = carPriceFull.replace('$', '');               
                                    var carPriceFull = carPriceFull.replace('*', '');               
                                    var carPriceFull = carPriceFull.replace(',', '');               
                                    var carPrice = carPriceFull.trim();
                                    var dealerAddress = "{addr1: '"+ addressFull.addr1 +"', city: '"+ addressFull.city +"', region: '"+ addressFull.region +"', postal_code: '"+ addressFull.postal_code +"', country: '"+ addressFull.country +"'}";
                                    var dealerLat = latLongFull.latitude;
                                    var dealerLong = latLongFull.longitude;
                                    var carColor = detail[gig].color;

                                    arrSetup = [carVin, carTitle, carDescrip, carURL, carMake, carModel, carYear, carMileage, 'MI', '', '', 'AUTOMATIC', 'GASOLINE', 'OTHER', 'Other', carVin, 'OTHER', carPrice + " USD", dealerAddress, carColor, carPrice + " USD", 'AVAILABLE', 'USED', dealerLat, dealerLong];
                                    newResults.push(arrSetup);
                                }
                                catch(error){
                                    returnString += error;
                                }
                            }
                            return newResults;

                        })
                        .catch(error => {
                            throw new Error(error);
                        });

                });             
            }, Promise.resolve([]))
                .then(function(finalCall){

                    /*
                        We need to get the 3rd image on every vdp in the array. We will need to create a loop, go to the page, get the image and properly insert it into the proper array index
                    */

                    finalCall.reduce(function(accumulator, resultArray){
                        return accumulator.then(function(finalResults){
                            var vdp = resultArray[3];
                            return nightmare.goto(vdp)
                                .wait(500)
                                .evaluate(() => {
                                    var thirdIMG = $('.gallery-thumbs .owl-item:nth-of-type(3) img').attr('src');
                                    return thirdIMG;
                                })
                                .then(imgResult => {
                                    // 9
                                    resultArray.splice(9, 1, imgResult);
                                    console.log(resultArray);
                                    finalResults.push(resultArray);
                                    return finalResults;
                                })
                                .catch(error => {
                                    throw new Error(error);
                                });

                        });
                    }, Promise.resolve([]))
                        .then(finalInsert => {
                            const csvWriter = createCsvWriter({
                            header: ["vehicle_id", "title", "description", "url", "make", "model", "year", "mileage.value", "mileage.unit", "image[0].url", "image[0].tag[0]", "transmission", "fuel_type", "body_style", "drivetrain", "vin", "condition", "price", "address", "exterior_color", "sale_price", "availability", "state_of_vehicle", "latitude", "longitude"],
                                path: 'test.csv'
                            });

                            var records = finalInsert;
                            console.log(records)
                            csvWriter.writeRecords(records)
                            .then(() => {
                                nightmare.end();
                                console.log('...Done');
                            });
                        })
                });
        })      
        .catch(function(error){
            return error;
        })
}

    getInventory(startURL, function(response){
        try {
            console.log("This is the response" + response);
        }
        catch(error){
            console.log(error)
        }
    });

---

不接受回调参数。只要归还承诺。@Bergi如果我理解正确的话，因为回调现在有点痛苦，你建议删除这些，只需要一个简单的返回语句。我试过了，但还是不行。这里很混乱。一个正确的返回语句，但是：-我想说的是，你不应该把回调风格和承诺风格混为一谈。不过，您可能应该先学习回调。感谢您的响应，但是我仍然会为循环中的每个getDetails调用返回未定义的结果。感谢您的响应，但似乎仍然返回未定义的结果。我从element.getAttribute改回使用jquery，因为第一次迭代导致未定义元素的错误。我还在努力。如果我解决了这个问题，我也会留下一个答案，以防这对将来的任何人都有帮助。@soulglow1985为什么要使用噩梦？是否要刮取在客户端上呈现react、angular、vue。。。？如果是这样，那么最好使用http请求并向数据api发出请求。如果站点提供html，您可以将http请求与jsdom或类似scrapy的东西一起使用，并行解析html。我认为问题在于调用了getDetails，而之前的getDetails尚未完成。已更新应答以连续调用getDetails。