Remove jQuery by default

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+- Start Date: 2018-10-07
+- RFC PR: https://github.com/emberjs/rfcs/pull/386
+- Ember Issue: (leave this empty)
+
+# Remove jQuery by default
+
+## Summary
+
+This RFC proposes deprecating those public APIs that are coupled to jQuery, and to finally remove them (with an optional
+backport), so Ember apps will be built *by default* without bundling jQuery. 
+
+While [RFC294](https://emberjs.github.io/rfcs/0294-optional-jquery.html), which is already implemented, provides
+a way to opt out of jQuery, the intention of this RFC is to push this a step further and essentially move from the
+current "included by default, allow opt out" strategy to "excluded by default, allow opt in".
+
+In that way it is not meant as a replacement of the previous RFC, but rather as a continuation and the logical next step.
+
+## Motivation
+
+### Lean by default
+
+This follows the philosophy of making Ember leaner (or *higher octane* if you want), by deprecating unused or 
+non-essential APIs.
+New apps will be smaller and faster by default, while allowing to opt-in into using jQuery when needed.
+
+### Why the current opt-out strategy is not sufficient
+
+The biggest problem in the current opt-out strategy is that many addons still require jQuery. Many of these usages
+seem to be rather "accidental", in that the full power of jQuery is not really needed for the given task, and could be 
+rather easily refactored to use only native DOM APIs. But as it is available anyway by default, and it is very convenient, 
+authors probably tend to use it without being fully aware of the consequences, that it prohibits jQuery-less builds for 
+all its consumers.
+
+In that way the general availability of jQuery *by default* and Ember APIs around it like `this.$()` tend to manifest the 
+status quo, the coupling of Ember to jQuery. In fact I could observe an actual *increase* of jQuery usage numbers
+(see below), rather than a decrease, which was an intention of the previous RFC. So it is not only a concern of the core 
+Ember library to enable jQuery-less builds, but the whole addon ecosystem has to go through that transition.
+
+In that regard early deprecations will help prevent this accidental use of jQuery on the one side, and on the other side
+for addons that depend on jQuery already they will provide an incentive and a long enough transition period to refactor 
+their jQuery usage to use standard DOM APIs.
+
+### jQuery might still be needed
+
+This RFC does not propose to discourage the use of jQuery. There are legitimate cases where you still want to have it. 
+And this is also true for addons, especially those that basically wrap other jQuery-based libraries like jQuery plugins
+in an Ember friendly way. For those cases, there should be an *opt-in* path to continue bundling jQuery and to preserve
+the existing APIs around it. This is what the `@ember/jquery` package is meant for.
+
+## Transition path
+
+### Add deprecations
+
+All current public APIs that are coupled to jQuery should be deprecated via the usual deprecation process. 
+This specifically involves:
+
+* adding a (universal, non-silenceable) deprecation warning to `Ember.$()`
+* adding a deprecation warning to `this.$()` in an `Ember.Component`
+* adding a deprecation warning to `this.$()` in component integration tests, based on `setupRenderingTest()` 
+
+### `this.$()` in old style tests
+
+`this.$()` in tests based on the old `moduleForComponent()` based testing APIs will not be specifically deprecated, 
+as these legacy testing APIs will eventually be deprecated altogether, as already envisaged in RFC232.
+
+### Extend `@ember/jquery` package
+
+For apps and addons that have to or choose to still require jQuery, they can add this package to its dependencies.
+This will provide a way to retain the deprecated and later removed APIs. So by adding this to your dependencies this 
+would effectively be the way to *opt-in* to require jQuery.
+
+RFC294 already introduced this package, being responsible to include jQuery into the JavaScript bundle. As part of this
+RFC the scope of this addon will be extended to also reintroduce the deprecated APIs, but *without* triggering any 
+deprecation warnings for `this.$()` in a component.
+
+As the default `EventDispatcher`, which currently dispatches jQuery events when jQuery is enabled, will eventually 
+support native events only (see the Timeline below), the addon also needs to replace it with one that again dispatches
+jQuery events for compatibility with existing jQuery-based code. This can happen in a similar way as 
+[ember-native-dom-event-dispatcher](https://github.com/rwjblue/ember-native-dom-event-dispatcher) did it, just the other
+way around.
+
+**This effectively makes the integration of jQuery a feature of this addon, rather than Ember itself, which is freed from
+the burden to care about this.**
+
+So effectively, for the Ember 3.x release cycle, adding this package will not change the behavior in any significant way,
+other than removing the mentioned deprecation warnings, as Ember will still have these APIs available. However starting 
+with Ember 4.0, which will have these APIs removed and not include jQuery integration features anymore, this 
+package will make sure jQuery remains included and it will add the now removed APIs back again, so any jQuery depending
+code will continue to work just as before. Also see the timeline below.
+
+As `ember-cli-babel` will currently transform `import $ from 'jquery';` to use `Ember.$` again, it must be made aware of
+the `@ember/jquery` package so it tells `babel-plugin-ember-modules-api-polyfill` not to convert those imports to the 
+global `Ember.$`. Instead the package itself should provide the necessary shim to make `import $ from 'jquery';` work.
+
+Addons that continue to depend on jQuery would have to list this package as a dependency in their `package.json`, 
+to make their consuming app automatically include jQuery and the related APIs in its bundle as mentioned above.
+Thereby they make their dependency on jQuery explicit, which in turn helps users to make an educated choice if they 
+deem this to be acceptable.
+
+### Extend ember-fetch
+
+The `ember-fetch` addon integrates the newer [`Fetch API`](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API)
+nicely into an Ember app, with an (optional) polyfill for older browsers. This can be used as a replacement for the
+jQuery-based `ember-ajax`. 
+
+One piece that is missing so far when switching is a convenient way to customize all outgoing requests, e.g. to add
+HTTP headers for authentication tokens. When using jQuery's AJAX implementation, this could be easily done using its
+[`prefilter`](http://api.jquery.com/jquery.ajaxprefilter/) function. To facilitate something similar when using 
+`ember-fetch`, the addon should be extended with an appropriate API, e.g. by adding a simple service through which 
+fetch requests are issued, which provides similar features for customization. The exact API of such a service is however
+out of scope for this RFC.
+
+### Make ember-data use ember-fetch
+
+It must be ensured that all parts of the core Ember experience work flawlessly without jQuery. Currently `ember-data`
+is still relying on jQuery for its XHR requests. By the time this RFC is implemented (i.e. the deprecation messages are 
+added), it must work out of the box without jQuery. 
+
+Fortunately [migration efforts](https://github.com/emberjs/data/pull/5386) are well advanced to support the `fetch` API
+through `ember-fetch`, so we can expect that to land soon enough that it does not block the transition.
+
+### Update app blueprint
+
+The blueprint to create a new app with `ember new` should be updated to not use jQuery by default. This involves to
+* disable jQuery integration by default (in `config/optional-features.json`)
+* remove the `@ember/jquery` package
+* replace `ember-ajax` with `ember-fetch`
+* add the [`no-jquery`](https://github.com/ember-cli/eslint-plugin-ember/blob/master/docs/rules/no-jquery.md) rule to the
+default ESLint config
+
+### Timeline
+
+During Ember 3.x:
+1. migrate the jQuery integration features to the `@ember/jquery` package
+2. update the blueprints as stated above
+3. add deprecation warnings as stated above
+
+Upon Ember 4.0
+* remove deprecated functions
+* remove the jQuery specific code paths in the `EventDispatcher`
+
+## How we teach this
+
+As part of the efforts to make jQuery optional, the guides have already been updated to have all examples teach native 
+DOM APIs instead of jQuery, and the new testing APIs. 
+The [jQuery migration guide](https://guides.emberjs.com/release/configuring-ember/optional-features/#toc_jquery-integration)
+already mentions the APIs that are not available anymore without jQuery and how to opt-out now.
+
+Activating the `no-jquery` ESLint rule will warn developers about any usages of the jQuery-based APIs being deprecated
+here. 
+
+The newly added deprecation messages should link to a deprecation guide, which will provide details on how to silence
+these deprecations, either by using native DOM APIs only or by installing `@ember/jquery` to explicitly opt-in into 
+jQuery. 
+
+For apps the tone of it should be neutral regarding jQuery itself, in the sense that using jQuery is neither
+bad nor good by itself. It depends on the context of the app if using jQuery makes sense or not. It is just that *Ember* 
+does no need it anymore, so it is not part of the default Ember experience anymore.
+
+For addons the story is a bit different, in that they are not aware of their app's context, so they should abstain from
+using jQuery if possible. See the [Motivation](#motivation) chapter above.
+
+## Drawbacks
+
+### Churn
+
+A vast amount of addons still depend on jQuery, so adding the deprecations will add some considerable churn for the addon
+ecosystem. As of writing this, there are:
+* [407 addons](https://emberobserver.com/code-search?codeQuery=Ember.%24) using `Ember.$`
+* [546 addons](https://emberobserver.com/code-search?codeQuery=this.%24&fileFilter=addon%2Fcomponents) using `this.$` in components
+* [994 addons](https://emberobserver.com/code-search?codeQuery=this.%24&fileFilter=tests) using `this.$` in tests
+
+A good amount of that churn can be mitigated by
+* existing codemods that migrate tests
+* having an easy way, given by the `@ember/jquery` package, to opt-in to continue bundling jQuery, and to restore the 
+deprecated APIs, so no further refactorings are required 
+
+## Alternatives
+
+Stick to the current *opt-out* process.
+