How do I handle JavaScript redirects when scraping with Kanna?

Kanna (formerly known as Swift HTML Parser) is a Swift library for parsing HTML and XML. It provides a way to select and manipulate HTML elements with an API similar to that of jQuery. When web scraping with Kanna, you might encounter JavaScript redirects, where a webpage uses JavaScript to navigate to a different URL.

Since Kanna itself does not execute JavaScript, it cannot directly handle JavaScript redirects. Instead, you'll need to detect these redirects and handle them within your Swift code or use additional tools that can execute JavaScript.

Here are some strategies to handle JavaScript redirects when scraping with Kanna:

1. Detect and Follow JavaScript Redirect Manually

You can look for the presence of JavaScript code that performs the redirect and then extract the URL to follow it manually. For example, you might look for window.location assignments in the script tags:

import Kanna

// Fetch the HTML content from the webpage
let html = // Fetch the HTML content using URLSession or any other method

// Parse the HTML with Kanna
if let doc = try? HTML(html: html, encoding: .utf8) {
    // Look for script tags and search for JavaScript redirects
    for script in doc.xpath("//script") {
        if let scriptText = script.text,
           let range = scriptText.range(of: "window.location.href = '") {
            let urlStartIndex = scriptText.index(range.upperBound, offsetBy: 0)
            if let urlEndIndex = scriptText[urlStartIndex...].firstIndex(of: "'") {
                let redirectUrl = String(scriptText[urlStartIndex..<urlEndIndex])
                // Now you have the redirect URL and can make another request to it
            }
        }
    }
}

2. Use a Headless Browser

To fully handle JavaScript, including JavaScript redirects, you can use a headless browser such as Puppeteer (for Node.js), Selenium, or Playwright. These tools can execute JavaScript and simulate a real browser environment.

Here is an example using Puppeteer in Node.js:

const puppeteer = require('puppeteer');

(async () => {
  const browser = await puppeteer.launch();
  const page = await browser.newPage();

  // By navigating to the URL, Puppeteer will automatically handle JavaScript redirects
  await page.goto('https://example.com/');

  // Once the page is loaded (after redirects), you can access the final URL if needed
  const finalUrl = page.url();

  // You can now scrape the content of the final page
  const content = await page.content();

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

3. Use Networking Libraries

Networking libraries can be used to follow redirects by examining the HTTP headers. However, this only works for server-side redirects (using HTTP 3xx status codes) and not for JavaScript-based redirects. If the redirect is implemented in JavaScript, you will still need to use one of the methods mentioned above.

4. Combine Kanna with a JavaScript Engine

You could combine Kanna with a JavaScript engine like JavaScriptCore (available on iOS and macOS) to evaluate the JavaScript code and find the redirect URL. This approach is more complex and would require you to load the JavaScript code into the engine and attempt to execute it, capturing the redirect URL.

Conclusion

If you're dealing with JavaScript redirects while web scraping with Kanna, you'll either need to detect and follow the redirects manually or use a tool that can execute JavaScript. Kanna itself is limited to parsing static HTML/XML content and does not have the capability to handle JavaScript execution.