This post aims at presenting how to beautify/unminify all the JavaScript files loaded in a page. The approach proposed uses Chrome headless instrumented by Puppeteer and the js-beautify library. Other approaches are possible. For example, one could use a simple HTTP library such as urllib.request in Python or the HTTP module in NodeJS and then parse the DOM to obtain the list of scripts included in the page in order to download all of them. While this alternative approach requires less CPU and RAM resources than running a whole headless browser, not all the pages can be properly crawled without JavaScript support. Moreover, in the case where you are already using a crawler based on a headless browser, it is a waste of time and bandwidth to load the scripts again since they have already been loaded by the crawler.

Example

The solution proposed in this post is based on Chrome headless instrumented with Puppeteer. Moreover, in our example, URLs are explored sequentially. Doing it at scale would require both making the crawler explore URLs in parallel, as well as managing errors properly to avoid crashes.

The first step consists in installing the Puppeteer and js-beautify packages using npm install. In the terminal run the following commands:

npm install puppeteer
npm install js-beautify

We import the libraries and create a function convertURLToFileName that is used later to hash a URL in order to obtain a file name.

const fs = require('fs');
const crypto = require('crypto');
const beautify = require('js-beautify').js
const puppeteer = require('puppeteer');

function convertURLToFileName(url) {
  return crypto.createHash('md5').update(url).digest("hex");
}

The second step consists in creating an instance of Chrome headless instrumented by Puppeteer. For each URL to crawl, we define a handler page.on(‘requestfinished’) to intercept all request whose type is script. Whenever a request corresponding to a script triggers the handler, we use the beautify function of the js-beautify library to beautify scripts on the fly.

(async () => {
  
  // List of URLs used in the example
  const urls = [
    'https://antoinevastel.com/bots',
    'https://antoinevastel.com',
  ];

  // We create an instance of Chrome headless
  const browser = await puppeteer.launch();

  for (const url of urls) {
    const page = await browser.newPage();
    const beautifiedFilePromises = [];
    page.on('requestfinished', async (interceptedRequest) => {
      // We intercept all requests whose type is 'script'
      if(interceptedRequest.resourceType() === 'script') {
        beautifiedFilePromises.push(new Promise(async (resolve) => {
          let redirectChain = interceptedRequest.redirectChain();
          if(redirectChain.length === 0) {
            let response = await interceptedRequest.response();
            const fileName = convertURLToFileName(interceptedRequest.url());
            if (response !== null) {
              // We obtain the content of the script
              let contentRequest = await response.text();

              // We use the beautify function from the js-beautify package
              // to beautify the code of the script
              const scriptBeautified = beautify(contentRequest, { 
                indent_size: 2, 
                space_in_empty_paren: true 
              });
              // We use 'writeFile' to save the beautified content to the disk asynchronously
              // We use promises that are stored in 'beautifiedFilePromises' to ensure 
              // all files have been written before we crawl another page 
              fs.writeFile(`files/${fileName}.js`, scriptBeautified, 'utf8', (err) => {
                if (err !== null) {
                  console.error(`Could not save the beautified file: ${err.message}`);
                }
                resolve();
              });
            }
          }
        })); 
      }
    });

    await page.goto(url);
    await Promise.all(beautifiedFilePromises);
    await page.close();
  }

  await browser.close();
})();

A complete working example is available on Github.


Antoine Vastel

French PhD student, working on browser fingerprinting!