回调函数Node JS
[英]Callback function Node JS
问题描述
我最近正在研究nodejs（并使用nightmare.js）来解析网站，并且在回调函数和显示返回的结果方面遇到问题。 我正在尝试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)
})
4 个解决方案
解决方案1
1 2018-02-06 14:43:54
我不会费心告诉您，不要将回调与这样的承诺混在一起。 但是，让我们现在来看问题。
情况1
您也如何检查错误？ 也许您的脚本抛出了错误。 我可以看到您在.catch上调用了回调.then但是在.catch上没有.catch 。 也许那时永远不会获取任何数据。
情况二
让我们检查一下您的功能。 您每次都呼叫.end 。 您是否也在每次都创建新的Nightmare实例？
在getInventory函数上，您不应调用.end 。 在getDetails函数上，您不应调用.end 。 它结束了噩梦实例，您正在丢失数据。
完成所有功能和工作后，调用nightmare.end() 。 要正确执行此操作，您将需要在下面了解有关Promise Check case 3的更多信息。
情况3
了解诺言如何运作。 在下面的行中，您无需等待函数完成。
getINV = getDetails(newURL, function(returnString){
callback(returnString)
})
您应该等待诺言完成。 另外，请确保不要在噩梦中尝试同时浏览两个链接。
因此，继续学习Promises和异步等待的东西。
我将如何解决您的代码？
我会使用Promise.all，.map和其他新功能。 这是为您完成的一些示例代码，请勿复制粘贴或直接运行该代码，请尝试理解为什么它与您的代码不同以及它的结果是什么。
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：
解决方案2
0 2018-02-05 17:04:52
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)
})
解决方案3
0 2018-02-05 18:37:30
也许以下方法会起作用，更改循环以减少和映射，取出jQuery并进行了一些小的更改。
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。
- 摆脱了回调，并使函数返回promise。
这是代码：
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)
);
解决方案4
0 已采纳 2018-02-14 21:17:49
使用异步函数学习promise和回调是一项艰巨的任务。 我能够使它与以下对象一起使用。 谢谢大家的帮助和指导。 每个答案都将我送往另一个兔子洞，最终找到了解决方案。
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)
}
});
回调函数节点js的回调
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