: 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

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).