TL:DR; Double-check your wiki's site scripts and your personal scripts
to ensure "document.write" is no longer used.

Hey all,

We have strongly discouraged for many years the use of synchronous
"document.write()" to inject additional HTML into the output stream.
Across MediaWiki core, extensions, and gadgets this hasn't worked since 2012.
With two legacy exceptions: the 'site' and 'user' modules. We always found a way
to continue support for those. But this is now going to be removed.

In upcoming ResourceLoader upgrades and performance improvements, we will
cease support for synchronous document-write, in the site and user modules.
Use of document-write requires MediaWiki to instruct the browser to pause its
rendering before the browser may proceed to parse and display a page to users.

Even though most scripts don't use this feature, the mere fact that we support it
is causing a measurable impact on page load performance.

Starting in 1.26wmf17 (released to wikis this week), ResourceLoader will be
fully asynchronous. This change is already live on the Beta Cluster. [1]
This means it is no longer possible for the site and user scripts to, with
document-write, pause the browser execution and insert additional HTML in
the initial output stream.

Removing an API does not necessarily mean removing a capability. If you
encounter any issues or can't find a simple upgrade path for an existing
script, please reach out on the mailing list. Below is summary of a few
typical use cases:

1. Loading scripts.

Instead of `document.write("<script src=url></script>");`
use `mw.loader.load(url);` instead.

2. Loading stylesheets.

Instead of `document.write("<link rel=stylesheet href=url/>");`
use `mw.loader.load(url, "text/css");` instead.

3. Creating elements.

Instead of `document.write("<div>....</div>");`, use:

    var nodes = $.parseHTML("<div>..</div>"); $('body').append(nodes);

Or something like:

    $("<div>").attr({ "id": "foo" }).appendTo("body");


Please take some time to look through your wiki's site scripts (MediaWiki:Common.js,
MediaWiki:Vector.js, etc.) and make sure document-write is no longer used.
You can also use the search engine. For example:

https://nl.wikipedia.org/w/index.php?search=document.write&ns8=1
https://commons.wikimedia.org/w/index.php?search=document.write&ns8=1

Search results from mwgrep on all public wikis:
https://phabricator.wikimedia.org/P1832

Check out the migration page for other deprecations and common issues you may encounter:
https://www.mediawiki.org/wiki/ResourceLoader/Migration_guide_(users)

== Further reading ==

The ResourceLoader improvements that led to this change are tracked under
https://phabricator.wikimedia.org/T107399.

Refer to the following workboards for other tasks in this area:
https://phabricator.wikimedia.org/tag/mediawiki-resourceloader/board/?order=priority
https://phabricator.wikimedia.org/tag/performance-team/board/?order=priority

Now that the site and user modules are primary citizens in the ResourceLoader landscape,
their states can be tracked with mw.loader. This solves long-outstanding issues such as
https://phabricator.wikimedia.org/T106736 which sometimes caused malfunctions
in Common.js to affect user gadgets, VisualEditor, and other site tools.

— Krinkle

[1] http://en.wikipedia.beta.wmflabs.org/


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