Does the Google spider render JavaScript?

Question

Does the Google spider knows how to render JavaScript, or only HTML?

Answer 1

Apparently it does, some, according to Matt Cutts:

"For a while, we were scanning within JavaScript, and we were looking for links. Google has gotten smarter about JavaScript and can execute some JavaScript. I wouldn't say that we execute all JavaScript, so there are some conditions in which we don't execute JavaScript. Certainly there are some common, well-known JavaScript things like Google Analytics, which you wouldn't even want to execute because you wouldn't want to try to generate phantom visits from Googlebot into your Google Analytics".

Answer 2

Google now officially processes JavaScript.

In order to solve this problem, we decided to try to understand pages by executing JavaScript. It’s hard to do that at the scale of the current web, but we decided that it’s worth it. We have been gradually improving how we do this for some time. In the past few months, our indexing system has been rendering a substantial number of web pages more like an average user’s browser with JavaScript turned on.

• Sometimes things don't go perfectly during rendering, which may negatively impact search results for your site. Here are a few potential issues, and – where possible, – how you can help prevent them from occurring:
• If resources like JavaScript or CSS in separate files are blocked (say, with robots.txt) so that Googlebot can’t retrieve them, our indexing systems won’t be able to see your site like an average user. We recommend allowing Googlebot to retrieve JavaScript and CSS so that your content can be indexed better. This is especially important for mobile websites, where external resources like CSS and JavaScript help our algorithms understand that the pages are optimized for mobile. If your web server is unable to handle the volume of crawl requests for resources, it may have a negative impact on our capability to render your pages. If you’d like to ensure that your pages can be rendered by Google, make sure your servers are able to handle crawl requests for resources.
• It's always a good idea to have your site degrade gracefully. This will help users enjoy your content even if their browser doesn't have compatible JavaScript implementations. It will also help visitors with JavaScript disabled or off, as well as search engines that can't execute JavaScript yet.
• Sometimes the JavaScript may be too complex or arcane for us to execute, in which case we can’t render the page fully and accurately.
• Some JavaScript removes content from the page rather than adding, which prevents us from indexing the content.

As a result of this they have deprecated their crawlable Ajax standard:

In 2009, we made a proposal to make AJAX pages crawlable. Back then, our systems were not able to render and understand pages that use JavaScript to present content to users. Because "crawlers … [were] not able to see any content … created dynamically," we proposed a set of practices that webmasters can follow in order to ensure that their AJAX-based applications are indexed by search engines.

Times have changed. Today, as long as you're not blocking Googlebot from crawling your JavaScript or CSS files, we are generally able to render and understand your web pages like modern browsers. To reflect this improvement, we recently updated our technical Webmaster Guidelines to recommend against disallowing Googlebot from crawling your site's CSS or JS files.

Since the assumptions for our 2009 proposal are no longer valid, we recommend following the principles of progressive enhancement. For example, you can use the History API pushState() to ensure accessibility for a wider range of browsers (and our systems).

Answer 3

It does, partly, as a quick Google search reveals:
http://blogs.forbes.com/velocity/2010/06/25/google-isnt-just-reading-your-links-its-now-running-your-code/

Still Google is relatively quiet on what the bot exactly understands.

Answer 4

Yes, Google has been getting much better at executing and understanding your scripts. However, Google recommends that everything either gracefully degrades or that you instead use progressive enhancement so that without JavaScript the website still works. There are options to get around this with services such as Prerender.io, however, this uses the AJAX crawling scheme which Google recently deprecated.

Answer 5

Yes, Google can render JavaScript, but it is still the best solution to have server side rendered version for best search performance/ranking (source). Source. See more about Google and JavaScript frameworks.