configure eslint to use es6 (#1652)

.eslintrc.json

@@ -1,10 +1,13 @@
 {
-	"plugins": ["es5"],
-	"extends": ["eslint:recommended", "plugin:es5/no-es2015"],
+	"parserOptions": {
+		"emcaVersion": 6
+	},
+	"extends": ["eslint:recommended"],
 	"root": true,
 	"env": {
 		"browser": true,
-		"jquery": true
+		"jquery": true,
+		"es6": true
 	},
 	"globals": {
 		"mw": "readonly",

DEVELOPER.md

@@ -2,22 +2,22 @@
 
 ## Reviewing and merging pull requests (WIP)
 
-Collaborators are encouraged to thoroughly review and [test](./CONTRIBUTING.md#testing-your-code) each pull request, including their own.  Unless urgent or obvious, it can be helpful to leave PRs open for folks to opine.
+Collaborators are encouraged to thoroughly review and [test](./CONTRIBUTING.md#testing-your-code) each pull request, including their own. Unless urgent or obvious, it can be helpful to leave PRs open for folks to opine.
 
 Things to watch out for:
 
 - Items and processes laid out in [CONTRIBUTING.md](./CONTRIBUTING.md) are followed.
-- Twinkle is meant to run on the latest weekly version of MediaWiki as rolled out every Thursday on the English Wikipedia.  Backwards compatibility is not guaranteed.
-- The goal is for Twinkle and Morebits to support the same [browsers that MediaWiki supports](https://www.mediawiki.org/wiki/Browser_compatibility).  In particular, collaborators should avoid [unsupported additions](https://kangax.github.io/compat-table/es6/) from ES6 (aka ES2015); `.includes` and `.find` are among the most likely to show up, although the jQuery `$.find()` is fine. Our ESLint configuration includes a [plugin](https://github.com/nkt/eslint-plugin-es5) that should catch most cases.
-- Certain positional jQuery selectors like `:first`, `:last`, and `:eq` were [deprecated in jQuery version 3.4.0](https://blog.jquery.com/2019/04/10/jquery-3-4-0-released/) and should probably not be reintroduced.  Instead, use methods like `.first()`, `.last()`, or `.eq()`.
+- Twinkle is meant to run on the latest weekly version of MediaWiki as rolled out every Thursday on the English Wikipedia. Backwards compatibility is not guaranteed.
+- The goal is for Twinkle and Morebits to support the same [browsers for which MediaWiki provides Grade A support](https://www.mediawiki.org/wiki/Browser_compatibility), except IE 11. The Twinkle gadget on enwiki is configured so that we can use up to JavaScript version ES6. However, due to the MediaWiki minifier, we must not use keywords from ES2016 or later, such as async/await and RegEx /s flag. New functions from ES2016 or later, such as Array.includes() should be okay since these will not break the minifier.
+- Certain positional jQuery selectors like `:first`, `:last`, and `:eq` were [deprecated in jQuery version 3.4.0](https://blog.jquery.com/2019/04/10/jquery-3-4-0-released/) and should probably not be reintroduced. Instead, use methods like `.first()`, `.last()`, or `.eq()`.
 
 ## Updating scripts on Wikipedia
 
 There are two ways to upload Twinkle scripts to Wikipedia or another destination. You can do it with a [Perl script](#synchronization-using-syncpl) (recommended) or [manually](#manual-synchronization).
 
 After the files are synced, ensure that [MediaWiki:Gadgets-definition][] contains the gadget definition found in [gadget.txt](./gadget.txt) (`sync.pl` will report its status). In addition to the `Twinkle` definition, the gadget installs the `morebits` library as a hidden gadget, making it efficiently available for other tools to use. `Twinkle-pagestyles` is a hidden [peer gadget](https://www.mediawiki.org/wiki/ResourceLoader/Migration_guide_(users)#Gadget_peers) of Twinkle. Before Twinkle has loaded, it adds space where the TW menu would go in the Vector skin, so that the top bar does not "jump".
 
-[select2][] is also uploaded as a hidden gadget for better menus and to take advantage of the Resource Loader over the [Toolforge CDN](https://tools.wmflabs.org/cdnjs/); it is done so under the [MIT license](https://github.com/select2/select2/blob/develop/LICENSE.md).  Loading via the ResourceLoader causes it to register as a nodejs/commonjs environment with `module.exports`, so a slight tweak has been made, eliminating that check.  Ideally, this will be handled differently (see [external libraries](https://www.mediawiki.org/wiki/ResourceLoader/Migration_guide_for_extension_developers#Special_case_of_external_libraries) and [T108655](https://phabricator.wikimedia.org/T108655).  As such, be careful when updating select2 from upstream.
+[select2][] is also uploaded as a hidden gadget for better menus and to take advantage of the Resource Loader over the [Toolforge CDN](https://tools.wmflabs.org/cdnjs/); it is done so under the [MIT license](https://github.com/select2/select2/blob/develop/LICENSE.md). Loading via the ResourceLoader causes it to register as a nodejs/commonjs environment with `module.exports`, so a slight tweak has been made, eliminating that check. Ideally, this will be handled differently (see [external libraries](https://www.mediawiki.org/wiki/ResourceLoader/Migration_guide_for_extension_developers#Special_case_of_external_libraries) and [T108655](https://phabricator.wikimedia.org/T108655). As such, be careful when updating select2 from upstream.
 
 ### Manual synchronization
 

gadget.txt

@@ -1,4 +1,4 @@
-* Twinkle[ResourceLoader|dependencies=ext.gadget.morebits,ext.gadget.select2,mediawiki.api,mediawiki.language|rights=autoconfirmed|type=general|peers=Twinkle-pagestyles]|Twinkle.js|Twinkle.css|twinklearv.js|twinklewarn.js|twinkleblock.js|friendlywelcome.js|friendlyshared.js|friendlytalkback.js|twinklespeedy.js|twinkleprod.js|twinklexfd.js|twinkleimage.js|twinkleprotect.js|friendlytag.js|twinklediff.js|twinkleunlink.js|twinklefluff.js|twinkledeprod.js|twinklebatchdelete.js|twinklebatchprotect.js|twinklebatchundelete.js|twinkleconfig.js
-* morebits[ResourceLoader|dependencies=mediawiki.user,mediawiki.util,mediawiki.Title,jquery.ui|hidden]|morebits.js|morebits.css
+* Twinkle[ResourceLoader|dependencies=ext.gadget.morebits,ext.gadget.select2,mediawiki.api,mediawiki.language|rights=autoconfirmed|type=general|peers=Twinkle-pagestyles|requiresES6]|Twinkle.js|Twinkle.css|twinklearv.js|twinklewarn.js|twinkleblock.js|friendlywelcome.js|friendlyshared.js|friendlytalkback.js|twinklespeedy.js|twinkleprod.js|twinklexfd.js|twinkleimage.js|twinkleprotect.js|friendlytag.js|twinklediff.js|twinkleunlink.js|twinklefluff.js|twinkledeprod.js|twinklebatchdelete.js|twinklebatchprotect.js|twinklebatchundelete.js|twinkleconfig.js
+* morebits[ResourceLoader|dependencies=mediawiki.user,mediawiki.util,mediawiki.Title,jquery.ui|hidden|requiresES6]|morebits.js|morebits.css
 * Twinkle-pagestyles[hidden|skins=vector,vector-2022]|Twinkle-pagestyles.css
 * select2[ResourceLoader|hidden]|select2.min.js|select2.min.css

package.json

@@ -15,7 +15,6 @@
 	},
 	"devDependencies": {
 		"eslint": "^7.32.0",
-		"eslint-plugin-es5": "^1.5.0",
 		"jest": "^27.0.6",
 		"mock-mediawiki": "^1.2.2",
 		"mwn": "^0.11.1"