Doc: ADR for replacing Mustache with ES6 strings

Add architectural decision record for replacing Mustache.js with ES6
template literals. Also, fix the date format on a previous ADR to be
consistent with the others.

Bug: T165036
Change-Id: I561ab4aaf6258e27cb12787b370407626547d48c

doc/adr/0009-utilize-browser-caching.md

@@ -1,6 +1,6 @@
 # 9. Utilize browser caching
 
-Date: 05/24/2017
+Date: 24/05/2017
 
 ## Status
 

doc/adr/0010-replace-mustache-js-with-template-literals.md

@@ -0,0 +1,88 @@
+# 10. Replace Mustache.js with Template Literals
+
+Date: 22/03/2018
+
+## Status
+
+Accepted
+
+## Context
+
+UI HTML templates are currently populated with variables by the [mustache.js]
+template system. This is consistent with other code in MediaWiki but adds an
+8.1 KiB dependency* for functionality that is only used in a few places.
+
+Given that ES6 template literals provide similar readability and are part of
+JavaScript itself, we considered this to be a favorable and sustainable
+alternative to Mustache templates. Additionally, although the usage of template
+strings requires transpilation, adding transpiling support enables other ES6
+syntaxes to be used such as let / const, arrow functions, and destructuring, all
+of which are considered language improvements that Extension:Popups can leverage
+in many areas.
+
+We used the [Babel transpiler] with [babel-preset-env] to translate only the
+necessary JavaScript from ES6 to ES5 for [grade A browsers]. The overhead for
+this functionality is nonzero but expected to diminish in time and always be
+less than the size of the Mustache.js dependency. Please note that while most
+ES6 syntaxes are supported, the transpiler does not provide polyfills for new
+APIs (e.g., `Array.prototype.includes()`) unless configured to do so via
+[babel-polyfill]. As polyfills add more overhead and are related but independent
+of syntax, API changes were not considered in this refactoring.
+
+We also realized that manual HTML escaping of template parameters would be a
+necessary part of this change. This functionality is built into the double-curly
+brace syntax of mustache.js but is now performed using [mw.html.escape()]. These
+calls are a blemish on the code but appear only in the templates themselves and
+would be replaced transparently in a UI library such as Preact which leverages
+JSX. We also anticipate that the template literal syntax would transition neatly
+to JSX. We don't know that Extension:Popups will ever want to use a UI library
+and accept these shortcomings may always exist.
+
+*As reported by [mw.loader.inspect()] on 22/03/2018.
+
+[mustache.js]: https://github.com/janl/mustache.js
+[Babel transpiler]: https://babeljs.io
+[babel-preset-env]: https://babeljs.io/docs/plugins/preset-env
+[grade A browsers]: https://www.mediawiki.org/wiki/Compatibility#Browsers
+[babel-polyfill]: https://babeljs.io/docs/usage/polyfill
+[mw.html.escape()]: https://www.mediawiki.org/wiki/ResourceLoader/Core_modules#mediaWiki.html
+[mw.loader.inspect()]: https://www.mediawiki.org/wiki/ResourceLoader/Core_modules#mw.loader.inspect
+
+## Decision
+
+We compared the sizes before (d35286a) and after (4281670) transpiling and they
+proved favorable:
+
+| Commit | index.js (gzip) |  index.js | ext.popups | ext.popups.main | ext.popups.images | mediawiki.template.mustache |      Total |
+| ------ | --------------: | --------: | ---------: | --------------: | ----------------: | --------------------------: | ---------: |
+| Before |       10.84 KiB | 32.88 KiB |       96 B |        52.5 KiB |           3.1 KiB |                     8.1 KiB | **65224B** |
+| After  |       11.46 KiB | 35.15 KiB |       96 B |        52.7 KiB |           3.1 KiB |                     0.0 KiB | **57193B** |
+
+Where "index.js (gzip)" is the minified gzipped size of the
+resources/dist/index.js Webpack build product as reported by [bundlesize],
+"index.js" is the minified uncompressed size of the same bundle as reported by
+[source-map-explorer] and [Webpack performance hints], and the remaining columns
+are the sum of minified uncompressed JavaScript and CSS sizes for each relevant
+module as reported by mw.loader.inspect() with the last column being a total of
+these inspect() modules.
+
+The conclusions to draw from this table are that transpilation does increase the
+size of the Webpack bundle but that the overhead is less than that of the
+Mustache.js dependency so the overall effect is a size improvement.
+Additionally, note that the transpiled bundle now encompasses the HTML templates
+which source-map-explorer reports as contributing a 2.53 KiB minified
+uncompressed portion of the 35.15 KiB bundle. (Previously, templates were an
+additional request.) Allowing for rounding errors, this brings the approximate
+overhead of enabling transpilation to nearly zero, 35.15 KiB - 32.88 KiB -
+2.53 KiB ≈ 0, which suggests transpiling as a viable solution for improving code
+elsewhere that must be written in modern form without compromising on
+compatibility or performance.
+
+[bundlesize]: https://github.com/siddharthkp/bundlesize
+[source-map-explorer]: https://github.com/danvk/source-map-explorer
+[Webpack performance hints]: https://webpack.js.org/configuration/performance
+
+## Consequences
+
+ES6 syntax is enabled without changing existing device support and 7.8 KiB less
+of minified uncompressed JavaScript is delivered.