The whole process of making a crawler with NodeJS (continued)_node.js
Continuing from the previous chapter, we need to modify the program to continuously capture the content of 40 pages. That is to say, we need to output the title, link, first comment, commenting user and forum points of each article.
As shown in the figure, the value obtained by $('.reply_author').eq(0).text().trim();
is the correct user who made the first comment.
{}
After eventproxy obtains the comments and username content, we need to jump to the user interface through the username to continue grabbing the user’s points
var $ = cheerio.load(topicHtml);
//This URL is the target URL for the next step
var userHref = 'https://cnodejs.org' $('.reply_author').eq(0).attr('href');
userHref = url.resolve(tUrl, userHref);
var title = $('.topic_full_title').text().trim().replace(/n/g,"");;
var href = topicUrl;
var comment1 = $('.reply_content').eq(0).text().trim();
var author1 = $('.reply_author').eq(0).text().trim();
//Pass parameters to the next concurrent crawl
ep.emit('user_html', [userHref, title, href, comment1, author1]);
In eventproxy this time, we need to find where the score is placed (class="big").
{}
It’s easy to find the classname. Let’s try to output the result first
var outcome = superagent.get(userUrl)
.end(function (err, res) {
If (err) {
return console.error(err);
}
var $ = cheerio.load(res.text);
var score = $('.big').text().trim();
console.log(user[1]);
console.log(user[2]);
console.log(user[3]);
console.log(user[4]);
console.log($('.big').text().trim());
return ({
title: user[1],
href: user[2],
comment1: user[3],
author1: user[4],
score1: score
});
});
});
Run the program and get the result of this code.
{}
But here comes the problem. We can correctly output the result in the callback function of .end(), but we cannot correctly output the outcome. If you look carefully, the outcome that needs to be output is a Request object. This is a careless mistake. The .end() function does not pass the return value to the Request object, and the result needs to be returned to the previous layer (users).
//find userDetails
ep.after('user_html', topicUrls.length, function(users){
users = users.map(function(user){
var userUrl = user[0];
var score;
superagent.get(userUrl)
.end(function (err, res) {
if (err) {
return console.error(err);
}
//console.log(res.text);
var $ = cheerio.load(res.text);
score = $('.big').text().trim();
});
return ({
title: user[1],
href: user[2],
comment1: user[3],
author1: user[4],
score1: score
});
});
把users好好地输出发现除了score1其他是正确值。仔细调试发现,程序是先进行了console.log(),然后再进行.map()。更准确地说,在.map()函数内,.get()的回调函数并没有执行完赋值score,return 返回值就进行了。这就是回调函数的异步,而外层的同步操作是不会等待回调函数做完操作的。
{}
我的做法就是eventproxy再emit一层消息,伴随着消息把需要的数据一起传递给接收消息操作.after(),只有当消息全部接收完毕,再打印出传递的参数(结果)。
score = $('.big')text().trim();
//新添加
ep.emit('got_score', [user[1], user[2], user[3], user[4], score]);
.....
ep.after('got_score', 10, function(users){
console.log(users);
});
{}
这个问题解决了,但score1的数值好像太大了点吧。再一看,原来class='big'有两个,用户的话题收藏也是属于这个class。我们得通过cheerio的.slice( start, [end] )来切取第一个元素,即将score 修改为 score = $('.big').slice(0).eq(0).text().trim();。正确结果如图。
{}

The differences in performance and efficiency between Python and JavaScript are mainly reflected in: 1) As an interpreted language, Python runs slowly but has high development efficiency and is suitable for rapid prototype development; 2) JavaScript is limited to single thread in the browser, but multi-threading and asynchronous I/O can be used to improve performance in Node.js, and both have advantages in actual projects.

JavaScript originated in 1995 and was created by Brandon Ike, and realized the language into C. 1.C language provides high performance and system-level programming capabilities for JavaScript. 2. JavaScript's memory management and performance optimization rely on C language. 3. The cross-platform feature of C language helps JavaScript run efficiently on different operating systems.

JavaScript runs in browsers and Node.js environments and relies on the JavaScript engine to parse and execute code. 1) Generate abstract syntax tree (AST) in the parsing stage; 2) convert AST into bytecode or machine code in the compilation stage; 3) execute the compiled code in the execution stage.

The future trends of Python and JavaScript include: 1. Python will consolidate its position in the fields of scientific computing and AI, 2. JavaScript will promote the development of web technology, 3. Cross-platform development will become a hot topic, and 4. Performance optimization will be the focus. Both will continue to expand application scenarios in their respective fields and make more breakthroughs in performance.

Both Python and JavaScript's choices in development environments are important. 1) Python's development environment includes PyCharm, JupyterNotebook and Anaconda, which are suitable for data science and rapid prototyping. 2) The development environment of JavaScript includes Node.js, VSCode and Webpack, which are suitable for front-end and back-end development. Choosing the right tools according to project needs can improve development efficiency and project success rate.

Yes, the engine core of JavaScript is written in C. 1) The C language provides efficient performance and underlying control, which is suitable for the development of JavaScript engine. 2) Taking the V8 engine as an example, its core is written in C, combining the efficiency and object-oriented characteristics of C. 3) The working principle of the JavaScript engine includes parsing, compiling and execution, and the C language plays a key role in these processes.

JavaScript is at the heart of modern websites because it enhances the interactivity and dynamicity of web pages. 1) It allows to change content without refreshing the page, 2) manipulate web pages through DOMAPI, 3) support complex interactive effects such as animation and drag-and-drop, 4) optimize performance and best practices to improve user experience.

C and JavaScript achieve interoperability through WebAssembly. 1) C code is compiled into WebAssembly module and introduced into JavaScript environment to enhance computing power. 2) In game development, C handles physics engines and graphics rendering, and JavaScript is responsible for game logic and user interface.


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