An update (March 2016) on the current state & recommendations for JavaScript sites / Progressive Web Apps [1] in Google Search. We occasionally see questions about what JS-based sites can do and still be visible in search, so here's a brief summary for today's state:

# Don't cloak to Googlebot. Use "feature detection" & "progressive enhancement" [2] techniques to make your content available to all users. Avoid redirecting to an "unsupported browser" page. Consider using a polyfill or other safe fallback where needed. The features Googlebot currently doesn't support include Service Workers, the Fetch API, Promises, and requestAnimationFrame.

# Use rel=canonical [3] when serving content from multiple URLs is required.

# Avoid the AJAX-Crawling scheme on new sites. Consider migrating old sites that use this scheme soon. Remember to remove "meta fragment" tags when migrating. Don't use a "meta fragment" tag if the "escaped fragment" URL doesn't serve fully rendered content. [4]

# Avoid using "#" in URLs (outside of "#!"). Googlebot rarely indexes URLs with "#" in them. Use "normal" URLs with path/filename/query-parameters instead, consider using the History API for navigation.

# Use Search Console's Fetch and Render tool [5] to test how Googlebot sees your pages. Note that this tool doesn't support "#!" or "#" URLs.

# Ensure that all required resources (including JavaScript files / frameworks, server responses, 3rd-party APIs, etc) aren't blocked by robots.txt. The Fetch and Render tool will list blocked resources discovered. If resources are uncontrollably blocked by robots.txt (e.g., 3rd-party APIs) or otherwise temporarily unavailable, ensure that your client-side code fails gracefully.

# Limit the number of embedded resources, in particular the number of JavaScript files and server responses required to render your page. A high number of required URLs can result in timeouts & rendering without these resources being available (e.g., some JavaScript files might not be loaded). Use reasonable HTTP caching directives.

# Google supports the use of JavaScript to provide titles, description & robots meta tags, structured data, and other meta-data. When using AMP, the AMP HTML page must be static as required by the spec, but the associated web page can be built using JS/PWA techniques. Remember to use a sitemap file with correct "lastmod" dates for signaling changes on your website.

# Finally, keep in mind that other search engines and web services accessing your content might not support JavaScript at all, or might support a different subset.

Looking at this list, none of these recommendations are completely new & limited to today -- and they'll continue to be valid for foreseeable future. Working with modern JavaScript frameworks for search can be a bit intimidating at first, but they open up some really neat possibilities to make fast & awesome sites!

I hope this was useful! Let me know if I missed anything, or if you need clarifications for any part.

-- update 2016-11-23 --

Here are some more resources that give some more insight into how we're crawling, rendering, and indexing modern sites:

https://webmasters.googleblog.com/2016/11/building-indexable-progressive-web-apps.html
https://www.youtube.com/watch?v=JlP5rBynK3E


Links:
[1] PWA: https://developers.google.com/web/progressive-web-apps
[2] Progressive enhancement: https://en.wikipedia.org/wiki/Progressive_enhancement
[3] rel=canonical: https://support.google.com/webmasters/answer/139066
[4] AJAX Crawling scheme: https://developers.google.com/webmasters/ajax-crawling/docs/specification
[5] https://support.google.com/webmasters/answer/6066468
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