How to write a crawler using JavaScript

With the continuous development of Internet technology, crawlers (Web Crawler) have become one of the most popular methods of crawling information. Through crawler technology, we can easily obtain data on the Internet and use it in many fields such as data analysis, mining, and modeling. The JavaScript language is gaining more and more attention because of its powerful front-end development tools. So, how to write a crawler using JavaScript? Next, this article will explain it to you in detail.

1. What is a crawler?

Crawler refers to an automated program that simulates the behavior of a browser to access various websites on the network and extract information from them. A crawler can generate a request to a website, get a corresponding response, and then extract the required information from the response. On the Internet, many websites provide API interfaces, but some websites do not provide such interfaces, so we need to use crawlers to grab the required data.

2. The principles and advantages of JavaScript crawlers

1. Principle

The principle of JavaScript crawlers is very simple. It mainly uses the Window object provided by the browser. Simulate the behavior of requesting a web page through the XMLHttpRequest or Fetch function, and then use the Document object to perform DOM operations to obtain the page DOM tree and extract useful information on the web page.

2. Advantages

Compared with other programming languages, the advantages of JavaScript crawlers are:

(1) Easy to learn and use

## The syntax of #JavaScript language is very concise and clear, and it is widely used in front-end development. Some of its methods and techniques are also applicable in web crawlers.

(2) Ability to achieve dynamic crawling

Some websites have anti-crawler mechanisms. For non-dynamic requests, the page may return an access denial message. Using JavaScript can simulate browser behavior, making it easier to crawl some dynamic websites.

(3) Wide application

JavaScript can run on multiple terminal devices and has a wide range of application scenarios.

3. The process of using JavaScript to write a crawler

To write a JavaScript crawler to obtain web page data, you need to follow the following process:

Send a request: the crawler will first Generate a URL and send an HTTP request to this URL to obtain the content of the web page to be crawled. This can be done using Ajax, fetch and other methods.
1. Get HTML content: The page resource has been downloaded. At this time, we need to parse the data in the HTML and obtain the DOM after parsing, so that we can perform subsequent operations on various data.
2. Parse data: Understand the data that needs to be crawled on the page, as well as the location and data type where this data appears on the page. You may need to use external libraries, such as jQuery, cheerio, htmlparser2 and other libraries, which can quickly parse page data.
3. Save data: You need to use File System to save the information we climbed down.

Below we use an example to explain the above process.

4. Learn how to write JavaScript crawlers through examples

In our example, we will use Node.js and jQuery, cheerio. The following is the website we will crawl: http://www.example.com

Install Node.js

If Node.js is not installed, you need to download Node first .js latest version. Run the following command to verify that Node.js is installed successfully.

node --version

If the installation is successful, the version number of Node.js will be displayed on the command line.

Create directories and files

Create a new directory locally and use the terminal to create a JavaScript file in that directory. For example, we create a directory named crawler and create a file named crawler.js in this directory.

Install jQuery and cheerio

We use lightweight jQuery in Node.js instead of native js to operate DOM (document), and use the cheerio module for DOM operations. Run the following commands to install the jQuery lightweight library and cheerio module.

npm install cheerio 
npm install jquery 

Writing JavaScript crawler code

In the crawler.js file, we write the following code.

Created a JavaScript file and imported two libraries cheerio and jQuery, which allow us to manipulate HTML content more conveniently. Next, create the express library and build the server. We retrieve the website and ask the cheerio module to load the HTML content into variables, then find the elements we are interested in in the HTML content and output them to the console.

The code is as follows:

// 导入库 
const cheerio = require('cheerio'); 
const express = require('express'); 
const request = require('request'); 

const app = express(); 

app.get('/', async (req, res, next) => { 
  try { 
    await request('http://www.example.com', (error, response, html) => { 
    
      const $ = cheerio.load(html); 
    
      const headings = $('h1'); 
    
      res.json(headings.text()); 
    }); 
  } catch (err) { 
    next(err); 
  } 
}); 

app.listen(3000); 

console.log('Server running at http://127.0.0.1:3000/');

Code analysis:

Request the HTML content of the http://www.example.com website through the get method of the request library, and the $ variable is cheerio Through this example, use $() to operate the DOM method and the HTML method to retrieve the H1 tag in the BODY tag. Use the res.json method to output our HTML content to the console.

Note:

The website content that the crawler needs to obtain must be public. If basic authentication is involved, the crawler cannot automatically obtain the data.
1. The speed of the crawler needs to be appropriate, and it is best not to be too fast, otherwise the server may think that you are accessing abnormally.

5. Summary

This article introduces how to use JavaScript to write crawlers as well as the advantages and principles. The advantage of JavaScript crawlers is that they are easy to learn and use, and can implement dynamic crawling. For dynamic website crawling, using JavaScript is very convenient and simple because of its cross-platform advantages and wide application. If you want to obtain data on the Internet and use it in data analysis, mining, modeling and other fields, JavaScript crawlers are a good choice.

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