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JavaScript script crawler is one of the most common crawling methods on the Internet. By executing JavaScript scripts, crawlers can automatically crawl data on the target website, process and store it. This article will introduce the principles, steps, and some practical techniques and tools of JavaScript script crawlers.
1. Principles of JavaScript script crawlers
Before introducing the principles of JavaScript script crawlers, let’s first understand JavaScript.
JavaScript is a scripting language usually used to write web page special effects and interactive operations. Unlike other programming languages, JavaScript is an interpreted language that does not require a compilation process and can be run directly in the browser. This feature allows JavaScript to quickly process and operate web page data.
The principle of JavaScript script crawler is to use JavaScript to perform web page data processing and operations, so as to achieve the purpose of crawling web page data.
2. JavaScript script crawler steps
After understanding the principle of JavaScript script crawler, you can start to understand the specific steps.
First you need to determine the target website to be crawled. Generally speaking, there are two types of websites crawled by crawlers: static websites and dynamic websites. A static website means that the data in the web page is already included in the HTML source code when requested, while a dynamic website dynamically generates and loads data through JavaScript. For static websites, you can directly parse the HTML source code for data processing and crawling; for dynamic websites, you need to use JavaScript to perform dynamic data processing and crawling.
After determining the target website, you need to carefully analyze the source code and data structure of the website. For static websites, it can be parsed through an HTML parser; for dynamic websites, you need to use a browser to simulate user access, and use browser developer tools to analyze the DOM structure and JavaScript code of the page.
Based on the analysis results, write JavaScript scripts to process and crawl website data. It should be noted that JavaScript scripts need to consider a variety of situations, such as asynchronous loading of the website, data paging, etc.
After writing the JavaScript script, it needs to be executed in the browser. JavaScript scripts can be loaded and executed through the console of the browser's developer tools.
After executing the JavaScript script, you can get the data on the website. Depending on the format and structure of the data, various data parsing tools can be used to parse it, and the parsed data can be saved to a local file or database.
3. JavaScript script crawler skills
In addition to the basic steps, there are also some practical skills that can help JavaScript script crawlers work more efficiently.
The web crawler framework can greatly simplify the crawler development process and improve development efficiency. Common JavaScript crawler frameworks include PhantomJS and Puppeteer.
When crawling websites, you need to be careful not to put too much burden on the target website, otherwise you may be banned from access by the website. At this time, a proxy IP can be used to hide the true source of access.
If you need to crawl data on the website regularly, you can use scheduled tasks to achieve automatic crawling. Common scheduled task tools include Cron and Node Schedule.
When crawling a website, you need to avoid too frequent requests to avoid excessive burden on the target website. You can use some techniques to limit the frequency of requests, such as setting the request interval or using crawler middleware.
4. JavaScript script crawler tools
When crawling JavaScript scripts, you can use some practical tools to improve development efficiency.
Chrome browser comes with powerful developer tools, including console, network tools, element inspector, etc., which can help Developers analyze the website's data structure and JavaScript code.
Node.js is a JavaScript-based development platform that can be used to write server-side and command-line tools. When crawling JavaScript scripts, you can use Node.js to execute JavaScript scripts and perform data parsing and processing.
Cheerio is a library similar to jQuery that can be used to parse the HTML source code of web pages and extract the required data. It supports selectors and executes very quickly, which can greatly simplify the process of data parsing.
Request is an HTTP request library that can be used to initiate HTTP requests and obtain responses. When crawling with JavaScript scripts, you can use Request to simulate user access to obtain website data.
Summarize
This article introduces the principles, steps, techniques and tools of JavaScript script crawlers. JavaScript script crawlers have the advantages of high flexibility and fast execution speed, providing an efficient and simple way to crawl website data. When using JavaScript script crawlers, you need to pay attention to comply with laws and regulations and the ethics of website vulnerability exploitation to avoid unnecessary losses to others or yourself.
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