Inject JavaScript with PhantomJS to inspect websites

By Matt Layman on March 24, 2015

How can you inspect the source of a website with a script? You can download the page, parse it with a library like lxml, and extract what you’re looking for. But what if the page is mostly made from JavaScript? There may not be much HTML to parse. Most of the page might be generated on the DOM dynamically. If this is the kind of page that you want to scan, a tool like lxml will not help much.

Another option is PhantomJS. PhantomJS is a “headless” web browser. That means that it is a browser that has no GUI. PhantomJS is exactly the kind of tool that you want for exploring very dynamic webpages from a script.

Let’s look at an example.

var page = require('webpage').create();
page.open('http://www.mattlayman.com', function(status) {
  if page.injectJs('scan.js') {
    var results = page.evaluate(function() {
        return getResults();
    });
    // Do something with the results from scan.js.
    phantom.exit();
  }
}

This little script opens http://www.mattlayman.com. When the page is loaded, the script injects a file named scan.js. This file contains whatever code you want. Maybe it is a bunch of jQuery to process the DOM. scan.js exists in the context of the page so it can access whatever data is needed. Then the script calls evaluate to invoke getResults. getResults is an imaginary method in scan.js. Data is transferred from the context of the website to the context of the PhantomJS script. With the scanning code done, you can do anything with the results.

PhantomJS makes it painless to get into a website, inspect the content of the site by looking at the DOM, and process the data to your heart’s content. This is a brief introduction, but I hope it gives you a flavor of what a headless browser like PhantomJS can do.

If you want to chat about this with me, I'm @mblayman on Twitter.



Matt Layman

Matt is the lead software engineer at Storybird.

Always eager to talk about Python and other technology topics, Matt organizes Python Frederick in Frederick, Maryland (NW of Washington D.C.) and seeks to grow software skills for people in his community.