Make JS async where possible

[davidfischer]

This will make analytics and footer JS load asynchronously. It is possible we see slightly reduced analytics from this but I doubt it will be significant. readthedocs-doc-embed.js already waits for document.ready before doing anything so I don't think there will be a significant effect there.

Because this is only available in the newer Sphinx API app.add_js_file, it will only apply if built with Sphinx 1.8+.

Resources

• MDN article on async
• Browser support for async (IE10+ and modern browsers)

[humitos]

Changes look good to me.

I tested this on a Bokeh build (https://bokeh-bokeh.readthedocs-hosted.com/en/readthedocs-async-js/) and I didn't find any problem. Although, the load speed was almost not increased. So, I'm not sure if we are going a bit faster with this or not.

Bokeh load time issue is another thing, though. Out of the scope of this PR.

I assume that _static/readthedocs-data.js is not possible to load in an async way, right?

[davidfischer]

I assume that _static/readthedocs-data.js is not possible to load in an async way, right?

Not currently. Ideally, I think this should be inlined rather than a separate file:

<script type="application/json" id="READTHEDOCS_DATA">{...}</script>

Anywhere that needs the data could just run JSON.parse(document.getElementById('READTHEDOCS_DATA'))

That's a bigger change though and would require a number of places to change. They'd also have to try the new way and fall back to the old way...