Writing a Performant Webcrawler in Javascript

Webcrawling, also known as web scraping, is the process of extracting data from a website. It can be a useful tool for data analysis, lead generation, and price comparison. However, it can also be a taxing process on both the webcrawler and the website being crawled. In this tutorial, we will go over how to write a performant webcrawler in javascript that is both efficient and respectful to the website being crawled.

Code

The first step in creating a webcrawler is to set up the basic structure of the script. We will be using the request and cheerio libraries to make HTTP requests and parse the HTML of the website, respectively.

const request = require('request');
const cheerio = require('cheerio');

Next, we will set up the function that will handle the webcrawling. The function will take in a URL as a parameter and use the request library to make an HTTP GET request to that URL.

function webcrawl(url) {
    request(url, (error, response, html) => {
        if (!error && response.statusCode == 200) {
            // success
        }
    });
}

Once the request is successful, we will use the cheerio library to parse the HTML and extract the data we need. In this example, we will extract all the links on the page.

function webcrawl(url) {
    request(url, (error, response, html) => {
        if (!error && response.statusCode == 200) {
            const $ = cheerio.load(html);
            $('a').each((i, link) => {
                console.log($(link).attr('href'));
            });
        }
    });
}

To make sure we are not overloading the website with requests, we will implement a delay between requests using the setTimeout function.

function webcrawl(url) {
    request(url, (error, response, html) => {
        if (!error && response.statusCode == 200) {
            const $ = cheerio.load(html);
            $('a').each((i, link) => {
                console.log($(link).attr('href'));
                setTimeout(() => {
                    webcrawl($(link).attr('href'));
                }, 1000);
            });
        }
    });
}

Explanation

The code above sets up a basic webcrawler that makes HTTP GET requests to a given URL and extracts all the links on the page. The request library is used to make the HTTP request and the cheerio library is used to parse the HTML and extract the data. The setTimeout function is used to implement a delay between requests to avoid overloading the website.

Conclusion

Writing a performant webcrawler in javascript is relatively simple with the use of libraries like request and cheerio. However it is important to keep in mind the performance and ethical